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* Fix some issues with improper placement of outer join clauses.Tom Lane2023-05-171-9/+12
| | | | | | | | | | | | | | | | | | | | | | | | | | After applying outer-join identity 3 in the forward direction, it was possible for the planner to mistakenly apply a qual clause from above the two outer joins at the now-lower join level. This can give the wrong answer, since a value that would get nulled by the now-upper join might not yet be null. To fix, when we perform such a transformation, consider that the now-lower join hasn't really completed the outer join it's nominally responsible for and thus its relid set should not include that OJ's relid (nor should its output Vars have that nullingrel bit set). Instead we add those bits when the now-upper join is performed. The existing rules for qual placement then suffice to prevent higher qual clauses from dropping below the now-upper join. There are a few complications from needing to consider transitive closures in case multiple pushdowns have happened, but all in all it's not a very complex patch. This is all new logic (from 2489d76c4) so no need to back-patch. The added test cases all have the same results as in v15. Tom Lane and Richard Guo Discussion: https://postgr.es/m/0b819232-4b50-f245-1c7d-c8c61bf41827@postgrespro.ru
* Fix mis-handling of outer join quals generated by EquivalenceClasses.Tom Lane2023-02-231-8/+42
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | It's possible, in admittedly-rather-contrived cases, for an eclass to generate a derived "join" qual that constrains the post-outer-join value(s) of some RHS variable(s) without mentioning the LHS at all. While the mechanisms were set up to work for this, we fell foul of the "get_common_eclass_indexes" filter installed by commit 3373c7155: it could decide that such an eclass wasn't relevant to the join, so that the required qual clause wouldn't get emitted there or anywhere else. To fix, apply get_common_eclass_indexes only at inner joins, where its rule is still valid. At an outer join, fall back to examining all eclasses that mention either input (or the OJ relid, though it should be impossible for an eclass to mention that without mentioning either input). Perhaps we can improve on that later, but the cost/benefit of adding more complexity to skip some irrelevant eclasses is dubious. To allow cheaply distinguishing outer from inner joins, pass the ojrelid to generate_join_implied_equalities as a separate argument. This also allows cleaning up some sloppiness that had crept into the definition of its join_relids argument, and it allows accurate calculation of nominal_join_relids for a child outer join. (The latter oversight seems not to have been a live bug, but it certainly could have caused problems in future.) Also fix what might be a live bug in check_index_predicates: it was being sloppy about what it passed to generate_join_implied_equalities. Per report from Richard Guo. Discussion: https://postgr.es/m/CAMbWs4-DsTBfOvXuw64GdFss2=M5cwtEhY=0DCS7t2gT7P6hSA@mail.gmail.com
* Fix some issues with wrong placement of pseudo-constant quals.Tom Lane2023-02-221-2/+2
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | initsplan.c figured that it could push Var-free qual clauses to the top of the current JoinDomain, which is okay in the abstract. But if the current domain is inside some outer join, and we later commute an inside-the-domain outer join with one outside it, we end up placing the pushed-up qual clause incorrectly. In distribute_qual_to_rels, avoid this by using the syntactic scope of the qual clause; with the exception that if we're in the top-level join domain we can still use the full query relid set, ensuring the resulting gating Result node goes to the top of the plan. (This is approximately as smart as the pre-v16 code was. Perhaps we can do better later, but it's not clear that such cases are worth a lot of sweat.) In process_implied_equality, we don't have a clear notion of syntactic scope, but we do have the results of SpecialJoinInfo construction. Thumb through those and remove any lower outer joins that might get commuted to above the join domain. Again, we can make an exception for the top-level join domain. It'd be possible to work harder here (for example, by keeping outer joins that aren't shown as potentially commutable), but I'm going to stop here for the moment. This issue has convinced me that the current representation of join domains probably needs further refinement, so I'm disinclined to write inessential dependent logic just yet. In passing, tighten the qualscope passed to process_implied_equality by generate_base_implied_equalities_no_const; there's no need for it to be larger than the rel we are currently considering. Tom Lane and Richard Guo, per report from Tender Wang. Discussion: https://postgr.es/m/CAHewXNk9eJ35ru5xATWioTV4+xZPHptjy9etdcNPjUfY9RQ+uQ@mail.gmail.com
* When removing a relation from the query, drop its RelOptInfo.Tom Lane2023-02-131-2/+1
| | | | | | | | | | | | | | | In commit b78f6264e I opined that it was "too risky" to delete a relation's RelOptInfo from the planner's data structures when we have realized that we don't need to join to it; so instead we just marked it as a dead relation. In hindsight that judgment seems flawed: any subsequent access to such a dead relation is arguably a bug in itself, so leaving the RelOptInfo present just helps to mask bugs. Let's delete it instead, allowing removal of the whole notion of a "dead relation". So far as the regression tests can find, this requires no other code changes, except for one Assert in equivclass.c that was very dubiously not complaining about access to a dead rel. Discussion: https://postgr.es/m/229905.1676062220@sss.pgh.pa.us
* Invent "join domains" to replace the below_outer_join hack.Tom Lane2023-01-301-53/+80
| | | | | | | | | | | | | | | | | | | | | | | | | | EquivalenceClasses are now understood as applying within a "join domain", which is a set of inner-joined relations (possibly underneath an outer join). We no longer need to treat an EC from below an outer join as a second-class citizen. I have hopes of eventually being able to treat outer-join clauses via EquivalenceClasses, by means of only applying deductions within the EC's join domain. There are still problems in the way of that, though, so for now the reconsider_outer_join_clause logic is still here. I haven't been able to get rid of RestrictInfo.is_pushed_down either, but I wonder if that could be recast using JoinDomains. I had to hack one test case in postgres_fdw.sql to make it still test what it was meant to, because postgres_fdw is inconsistent about how it deals with quals containing non-shippable expressions; see https://postgr.es/m/1691374.1671659838@sss.pgh.pa.us. That should be improved, but I don't think it's within the scope of this patch series. Patch by me; thanks to Richard Guo for review. Discussion: https://postgr.es/m/830269.1656693747@sss.pgh.pa.us
* Do assorted mop-up in the planner.Tom Lane2023-01-301-123/+45
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Remove RestrictInfo.nullable_relids, along with a good deal of infrastructure that calculated it. One use-case for it was in join_clause_is_movable_to, but we can now replace that usage with a check to see if the clause's relids include any outer join that can null the target relation. The other use-case was in join_clause_is_movable_into, but that test can just be dropped entirely now that the clause's relids include outer joins. Furthermore, join_clause_is_movable_into should now be accurate enough that it will accept anything returned by generate_join_implied_equalities, so we can restore the Assert that was diked out in commit 95f4e59c3. Remove the outerjoin_delayed mechanism. We needed this before to prevent quals from getting evaluated below outer joins that should null some of their vars. Now that we consider varnullingrels while placing quals, that's taken care of automatically, so throw the whole thing away. Teach remove_useless_result_rtes to also remove useless FromExprs. Having done that, the delay_upper_joins flag serves no purpose any more and we can remove it, largely reverting 11086f2f2. Use constant TRUE for "dummy" clauses when throwing back outer joins. This improves on a hack I introduced in commit 6a6522529. If we have a left-join clause l.x = r.y, and a WHERE clause l.x = constant, we generate r.y = constant and then don't really have a need for the join clause. But we must throw the join clause back anyway after marking it redundant, so that the join search heuristics won't think this is a clauseless join and avoid it. That was a kluge introduced under time pressure, and after looking at it I thought of a better way: let's just introduce constant-TRUE "join clauses" instead, and get rid of them at the end. This improves the generated plans for such cases by not having to test a redundant join clause. We can also get rid of the ugly hack used to mark such clauses as redundant for selectivity estimation. Patch by me; thanks to Richard Guo for review. Discussion: https://postgr.es/m/830269.1656693747@sss.pgh.pa.us
* Make Vars be outer-join-aware.Tom Lane2023-01-301-31/+101
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Traditionally we used the same Var struct to represent the value of a table column everywhere in parse and plan trees. This choice predates our support for SQL outer joins, and it's really a pretty bad idea with outer joins, because the Var's value can depend on where it is in the tree: it might go to NULL above an outer join. So expression nodes that are equal() per equalfuncs.c might not represent the same value, which is a huge correctness hazard for the planner. To improve this, decorate Var nodes with a bitmapset showing which outer joins (identified by RTE indexes) may have nulled them at the point in the parse tree where the Var appears. This allows us to trust that equal() Vars represent the same value. A certain amount of klugery is still needed to cope with cases where we re-order two outer joins, but it's possible to make it work without sacrificing that core principle. PlaceHolderVars receive similar decoration for the same reason. In the planner, we include these outer join bitmapsets into the relids that an expression is considered to depend on, and in consequence also add outer-join relids to the relids of join RelOptInfos. This allows us to correctly perceive whether an expression can be calculated above or below a particular outer join. This change affects FDWs that want to plan foreign joins. They *must* follow suit when labeling foreign joins in order to match with the core planner, but for many purposes (if postgres_fdw is any guide) they'd prefer to consider only base relations within the join. To support both requirements, redefine ForeignScan.fs_relids as base+OJ relids, and add a new field fs_base_relids that's set up by the core planner. Large though it is, this commit just does the minimum necessary to install the new mechanisms and get check-world passing again. Follow-up patches will perform some cleanup. (The README additions and comments mention some stuff that will appear in the follow-up.) Patch by me; thanks to Richard Guo for review. Discussion: https://postgr.es/m/830269.1656693747@sss.pgh.pa.us
* Update copyright for 2023Bruce Momjian2023-01-021-1/+1
| | | | Backpatch-through: 11
* Avoid making commutatively-duplicate clauses in EquivalenceClasses.Tom Lane2022-10-271-8/+20
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | When we decide we need to make a derived clause equating a.x and b.y, we already will re-use a previously-made clause "a.x = b.y". But we might instead have "b.y = a.x", which is perfectly usable because equivclass.c has never promised anything about the operand order in clauses it builds. Saving construction of a new RestrictInfo doesn't matter all that much in itself --- but because we cache selectivity estimates and so on per-RestrictInfo, there's a possibility of saving a fair amount of duplicative effort downstream. Hence, check for commutative matches as well as direct ones when seeing if we have a pre-existing clause. This changes the visible clause order in several regression test cases, but they're all clearly-insignificant changes. Checking for the reverse operand order is simple enough, but if we wanted to check for operator OID match we'd need to call get_commutator here, which is not so cheap. I concluded that we don't really need the operator check anyway, so I just removed it. It's unlikely that an opfamily contains more than one applicable operator for a given pair of operand datatypes; and if it does they had better give the same answers, so there seems little need to insist that we use exactly the one select_equality_operator chose. Using the current core regression suite as a test case, I see this change reducing the number of new join clauses built by create_join_clause from 9673 to 5142 (out of 26652 calls). So not quite 50% savings, but pretty close to it. Discussion: https://postgr.es/m/78062.1666735746@sss.pgh.pa.us
* Revert "Optimize order of GROUP BY keys".Tom Lane2022-10-031-12/+1
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | This reverts commit db0d67db2401eb6238ccc04c6407a4fd4f985832 and several follow-on fixes. The idea of making a cost-based choice of the order of the sorting columns is not fundamentally unsound, but it requires cost information and data statistics that we don't really have. For example, relying on procost to distinguish the relative costs of different sort comparators is pretty pointless so long as most such comparator functions are labeled with cost 1.0. Moreover, estimating the number of comparisons done by Quicksort requires more than just an estimate of the number of distinct values in the input: you also need some idea of the sizes of the larger groups, if you want an estimate that's good to better than a factor of three or so. That's data that's often unknown or not very reliable. Worse, to arrive at estimates of the number of calls made to the lower-order-column comparison functions, the code needs to make estimates of the numbers of distinct values of multiple columns, which are necessarily even less trustworthy than per-column stats. Even if all the inputs are perfectly reliable, the cost algorithm as-implemented cannot offer useful information about how to order sorting columns beyond the point at which the average group size is estimated to drop to 1. Close inspection of the code added by db0d67db2 shows that there are also multiple small bugs. These could have been fixed, but there's not much point if we don't trust the estimates to be accurate in-principle. Finally, the changes in cost_sort's behavior made for very large changes (often a factor of 2 or so) in the cost estimates for all sorting operations, not only those for multi-column GROUP BY. That naturally changes plan choices in many situations, and there's precious little evidence to show that the changes are for the better. Given the above doubts about whether the new estimates are really trustworthy, it's hard to summon much confidence that these changes are better on the average. Since we're hard up against the release deadline for v15, let's revert these changes for now. We can always try again later. Note: in v15, I left T_PathKeyInfo in place in nodes.h even though it's unreferenced. Removing it would be an ABI break, and it seems a bit late in the release cycle for that. Discussion: https://postgr.es/m/TYAPR01MB586665EB5FB2C3807E893941F5579@TYAPR01MB5866.jpnprd01.prod.outlook.com
* Improve performance of adjust_appendrel_attrs_multilevel.Tom Lane2022-08-181-8/+8
| | | | | | | | | | | | | | | | | | | | | | | | The present implementations of adjust_appendrel_attrs_multilevel and its sibling adjust_child_relids_multilevel are very messy, because they work by reconstructing the relids of the child's immediate parent and then seeing if that's bms_equal to the relids of the target parent. Aside from being quite inefficient, this will not work with planned future changes to make joinrels' relid sets contain outer-join relids in addition to baserels. The whole thing can be solved at a stroke by adding explicit parent and top_parent links to child RelOptInfos, and making these functions work with RelOptInfo pointers instead of relids. Doing that is simpler for most callers, too. In my original version of this patch, I got rid of RelOptInfo.top_parent_relids on the grounds that it was now redundant. However, that adds a lot of code churn in places that otherwise would not need changing, and arguably the extra indirection needed to fetch top_parent->relids in those places costs something. So this version leaves that field in place. Discussion: https://postgr.es/m/553080.1657481916@sss.pgh.pa.us
* Use an explicit state flag to control PlaceHolderInfo creation.Tom Lane2022-08-171-1/+1
| | | | | | | | | | | | | | | | | | | Up to now, callers of find_placeholder_info() were required to pass a flag indicating if it's OK to make a new PlaceHolderInfo. That'd be fine if the callers had free choice, but they do not. Once we begin deconstruct_jointree() it's no longer OK to make more PHIs; while callers before that always want to create a PHI if it's not there already. So there's no freedom of action, only the opportunity to cause bugs by creating PHIs too late. Let's get rid of that in favor of adding a state flag PlannerInfo.placeholdersFrozen, which we can set at the point where it's no longer OK to make more PHIs. This patch also simplifies a couple of call sites that were using complicated logic to avoid calling find_placeholder_info() as much as possible. Now that that lookup is O(1) thanks to the previous commit, the extra bitmap manipulations are probably a net negative. Discussion: https://postgr.es/m/1405792.1660677844@sss.pgh.pa.us
* Fix incorrect tests for SRFs in relation_can_be_sorted_early().Tom Lane2022-08-031-2/+2
| | | | | | | | | | | | | | | | | | | | | | | | | Commit fac1b470a thought we could check for set-returning functions by testing only the top-level node in an expression tree. This is wrong in itself, and to make matters worse it encouraged others to make the same mistake, by exporting tlist.c's special-purpose IS_SRF_CALL() as a widely-visible macro. I can't find any evidence that anyone's taken the bait, but it was only a matter of time. Use expression_returns_set() instead, and stuff the IS_SRF_CALL() genie back in its bottle, this time with a warning label. I also added a couple of cross-reference comments. After a fair amount of fooling around, I've despaired of making a robust test case that exposes the bug reliably, so no test case here. (Note that the test case added by fac1b470a is itself broken, in that it doesn't notice if you remove the code change. The repro given by the bug submitter currently doesn't fail either in v15 or HEAD, though I suspect that may indicate an unrelated bug.) Per bug #17564 from Martijn van Oosterhout. Back-patch to v13, as the faulty patch was. Discussion: https://postgr.es/m/17564-c7472c2f90ef2da3@postgresql.org
* Pre-beta mechanical code beautification.Tom Lane2022-05-121-3/+3
| | | | | Run pgindent, pgperltidy, and reformat-dat-files. I manually fixed a couple of comments that pgindent uglified.
* Fix comments with "a expression"Andrew Dunstan2022-03-311-1/+1
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* Fix postgres_fdw to check shippability of sort clauses properly.Tom Lane2022-03-311-29/+0
| | | | | | | | | | | | | | | | | | | | | | | | | postgres_fdw would push ORDER BY clauses to the remote side without verifying that the sort operator is safe to ship. Moreover, it failed to print a suitable USING clause if the sort operator isn't default for the sort expression's type. The net result of this is that the remote sort might not have anywhere near the semantics we expect, which'd be disastrous for locally-performed merge joins in particular. We addressed similar issues in the context of ORDER BY within an aggregate function call in commit 7012b132d, but failed to notice that query-level ORDER BY was broken. Thus, much of the necessary logic already existed, but it requires refactoring to be usable in both cases. Back-patch to all supported branches. In HEAD only, remove the core code's copy of find_em_expr_for_rel, which is no longer used and really should never have been pushed into equivclass.c in the first place. Ronan Dunklau, per report from David Rowley; reviews by David Rowley, Ranier Vilela, and myself Discussion: https://postgr.es/m/CAApHDvr4OeC2DBVY--zVP83-K=bYrTD7F8SZDhN4g+pj2f2S-A@mail.gmail.com
* Optimize order of GROUP BY keysTomas Vondra2022-03-311-1/+12
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | When evaluating a query with a multi-column GROUP BY clause using sort, the cost may be heavily dependent on the order in which the keys are compared when building the groups. Grouping does not imply any ordering, so we're allowed to compare the keys in arbitrary order, and a Hash Agg leverages this. But for Group Agg, we simply compared keys in the order as specified in the query. This commit explores alternative ordering of the keys, trying to find a cheaper one. In principle, we might generate grouping paths for all permutations of the keys, and leave the rest to the optimizer. But that might get very expensive, so we try to pick only a couple interesting orderings based on both local and global information. When planning the grouping path, we explore statistics (number of distinct values, cost of the comparison function) for the keys and reorder them to minimize comparison costs. Intuitively, it may be better to perform more expensive comparisons (for complex data types etc.) last, because maybe the cheaper comparisons will be enough. Similarly, the higher the cardinality of a key, the lower the probability we’ll need to compare more keys. The patch generates and costs various orderings, picking the cheapest ones. The ordering of group keys may interact with other parts of the query, some of which may not be known while planning the grouping. E.g. there may be an explicit ORDER BY clause, or some other ordering-dependent operation, higher up in the query, and using the same ordering may allow using either incremental sort or even eliminate the sort entirely. The patch generates orderings and picks those minimizing the comparison cost (for various pathkeys), and then adds orderings that might be useful for operations higher up in the plan (ORDER BY, etc.). Finally, it always keeps the ordering specified in the query, on the assumption the user might have additional insights. This introduces a new GUC enable_group_by_reordering, so that the optimization may be disabled if needed. The original patch was proposed by Teodor Sigaev, and later improved and reworked by Dmitry Dolgov. Reviews by a number of people, including me, Andrey Lepikhov, Claudio Freire, Ibrar Ahmed and Zhihong Yu. Author: Dmitry Dolgov, Teodor Sigaev, Tomas Vondra Reviewed-by: Tomas Vondra, Andrey Lepikhov, Claudio Freire, Ibrar Ahmed, Zhihong Yu Discussion: https://postgr.es/m/7c79e6a5-8597-74e8-0671-1c39d124c9d6%40sigaev.ru Discussion: https://postgr.es/m/CA%2Bq6zcW_4o2NC0zutLkOJPsFt80megSpX_dVRo6GK9PC-Jx_Ag%40mail.gmail.com
* Update copyright for 2022Bruce Momjian2022-01-071-1/+1
| | | | Backpatch-through: 10
* Rename find_em_expr_usable_for_sorting_rel.Tom Lane2021-04-201-12/+15
| | | | | | | | | | | I didn't particularly like this function name, as it fails to express what's going on. Also, returning the sort expression alone isn't too helpful --- typically, a caller would also need some other fields of the EquivalenceMember. But the sole caller really only needs a bool result, so let's make it "bool relation_can_be_sorted_early()". Discussion: https://postgr.es/m/91f3ec99-85a4-fa55-ea74-33f85a5c651f@swarm64.com
* Fix planner failure in some cases of sorting by an aggregate.Tom Lane2021-04-201-43/+212
| | | | | | | | | | | | | | | | | | | | | | | An oversight introduced by the incremental-sort patches caused "could not find pathkey item to sort" errors in some situations where a sort key involves an aggregate or window function. The basic problem here is that find_em_expr_usable_for_sorting_rel isn't properly modeling what prepare_sort_from_pathkeys will do later. Rather than hoping we can keep those functions in sync, let's refactor so that they actually share the code for identifying a suitable sort expression. With this refactoring, tlist.c's tlist_member_ignore_relabel is unused. I removed it in HEAD but left it in place in v13, in case any extensions are using it. Per report from Luc Vlaming. Back-patch to v13 where the problem arose. James Coleman and Tom Lane Discussion: https://postgr.es/m/91f3ec99-85a4-fa55-ea74-33f85a5c651f@swarm64.com
* Fix pull_varnos' miscomputation of relids set for a PlaceHolderVar.Tom Lane2021-01-211-6/+11
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Previously, pull_varnos() took the relids of a PlaceHolderVar as being equal to the relids in its contents, but that fails to account for the possibility that we have to postpone evaluation of the PHV due to outer joins. This could result in a malformed plan. The known cases end up triggering the "failed to assign all NestLoopParams to plan nodes" sanity check in createplan.c, but other symptoms may be possible. The right value to use is the join level we actually intend to evaluate the PHV at. We can get that from the ph_eval_at field of the associated PlaceHolderInfo. However, there are some places that call pull_varnos() before the PlaceHolderInfos have been created; in that case, fall back to the conservative assumption that the PHV will be evaluated at its syntactic level. (In principle this might result in missing some legal optimization, but I'm not aware of any cases where it's an issue in practice.) Things are also a bit ticklish for calls occurring during deconstruct_jointree(), but AFAICS the ph_eval_at fields should have reached their final values by the time we need them. The main problem in making this work is that pull_varnos() has no way to get at the PlaceHolderInfos. We can fix that easily, if a bit tediously, in HEAD by passing it the planner "root" pointer. In the back branches that'd cause an unacceptable API/ABI break for extensions, so leave the existing entry points alone and add new ones with the additional parameter. (If an old entry point is called and encounters a PHV, it'll fall back to using the syntactic level, again possibly missing some valid optimization.) Back-patch to v12. The computation is surely also wrong before that, but it appears that we cannot reach a bad plan thanks to join order restrictions imposed on the subquery that the PlaceHolderVar came from. The error only became reachable when commit 4be058fe9 allowed trivial subqueries to be collapsed out completely, eliminating their join order restrictions. Per report from Stephan Springl. Discussion: https://postgr.es/m/171041.1610849523@sss.pgh.pa.us
* Update copyright for 2021Bruce Momjian2021-01-021-1/+1
| | | | Backpatch-through: 9.5
* Improve find_em_expr_usable_for_sorting_rel commentTomas Vondra2020-12-221-3/+5
| | | | | | | | | | | Clarify the relationship between find_em_expr_usable_for_sorting_rel and prepare_sort_from_pathkeys, i.e. what restrictions need to be shared between those two places. Author: James Coleman Reviewed-by: Tomas Vondra Backpatch-through: 13 Discussion: https://postgr.es/m/CAAaqYe8cK3g5CfLC4w7bs%3DhC0mSksZC%3DH5M8LSchj5e5OxpTAg%40mail.gmail.com
* Don't search for volatile expr in find_em_expr_usable_for_sorting_relTomas Vondra2020-12-211-23/+5
| | | | | | | | | | | | While prepare_sort_from_pathkeys has to be concerned about matching up a volatile expression to the proper tlist entry, we don't need to do that in find_em_expr_usable_for_sorting_rel becausee such a sort will have to be postponed anyway. Author: James Coleman Reviewed-by: Tomas Vondra Backpatch-through: 13 Discussion: https://postgr.es/m/CAAaqYe8cK3g5CfLC4w7bs%3DhC0mSksZC%3DH5M8LSchj5e5OxpTAg%40mail.gmail.com
* Disallow SRFs when considering sorts below Gather MergeTomas Vondra2020-12-211-0/+7
| | | | | | | | | | | | | | | | | While we do allow SRFs in ORDER BY, scan/join processing should not consider such cases - such sorts should only happen via final Sort atop a ProjectSet. So make sure we don't try adding such sorts below Gather Merge, just like we do for expressions that are volatile and/or not parallel safe. Backpatch to PostgreSQL 13, where this code was introduced as part of the Incremental Sort patch. Author: James Coleman Reviewed-by: Tomas Vondra Backpatch-through: 13 Discussion: https://postgr.es/m/CAAaqYe8cK3g5CfLC4w7bs=hC0mSksZC=H5M8LSchj5e5OxpTAg@mail.gmail.com Discussion: https://postgr.es/m/295524.1606246314%40sss.pgh.pa.us
* Check parallel safety in generate_useful_gather_pathsTomas Vondra2020-12-211-1/+8
| | | | | | | | | | | | | | | | | | Commit ebb7ae839d ensured we ignore pathkeys with volatile expressions when considering adding a sort below a Gather Merge. Turns out we need to care about parallel safety of the pathkeys too, otherwise we might try sorting e.g. on results of a correlated subquery (as demonstrated by a report from Luis Roberto). Initial investigation by Tom Lane, patch by James Coleman. Backpatch to 13, where the code was instroduced (as part of Incremental Sort). Reported-by: Luis Roberto Author: James Coleman Reviewed-by: Tomas Vondra Backpatch-through: 13 Discussion: https://postgr.es/m/622580997.37108180.1604080457319.JavaMail.zimbra%40siscobra.com.br Discussion: https://postgr.es/m/CAAaqYe8cK3g5CfLC4w7bs=hC0mSksZC=H5M8LSchj5e5OxpTAg@mail.gmail.com
* Fix get_useful_pathkeys_for_relation for volatile expressionsTomas Vondra2020-11-031-0/+70
| | | | | | | | | | | | | | When considering Incremental Sort below a Gather Merge, we need to be a bit more careful when matching pathkeys to EC members. It's not enough to find a member whose Vars are all in the current relation's target; volatile expressions in particular need to be contained in the target, otherwise it's too early to use the pathkey. Reported-by: Jaime Casanova Author: James Coleman Reviewed-by: Tomas Vondra Backpatch-through: 13, where the incremental sort code was added Discussion: https://postgr.es/m/CAJGNTeNaxpXgBVcRhJX%2B2vSbq%2BF2kJqGBcvompmpvXb7pq%2BoFA%40mail.gmail.com
* Fix foreign-key selectivity estimation in the presence of constants.Tom Lane2020-10-281-25/+96
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | get_foreign_key_join_selectivity() looks for join clauses that equate the two sides of the FK constraint. However, if we have a query like "WHERE fktab.a = pktab.a and fktab.a = 1", it won't find any such join clause, because equivclass.c replaces the given clauses with "fktab.a = 1 and pktab.a = 1", which can be enforced at the scan level, leaving nothing to be done for column "a" at the join level. We can fix that expectation without much trouble, but then a new problem arises: applying the foreign-key-based selectivity rule produces a rowcount underestimate, because we're effectively double-counting the selectivity of the "fktab.a = 1" clause. So we have to cancel that selectivity out of the estimate. To fix, refactor process_implied_equality() so that it can pass back the new RestrictInfo to its callers in equivclass.c, allowing the generated "fktab.a = 1" clause to be saved in the EquivalenceClass's ec_derives list. Then it's not much trouble to dig out the relevant RestrictInfo when we need to adjust an FK selectivity estimate. (While at it, we can also remove the expensive use of initialize_mergeclause_eclasses() to set up the new RestrictInfo's left_ec and right_ec pointers. The equivclass.c code can set those basically for free.) This seems like clearly a bug fix, but I'm hesitant to back-patch it, first because there's some API/ABI risk for extensions and second because we're usually loath to destabilize plan choices in stable branches. Per report from Sigrid Ehrenreich. Discussion: https://postgr.es/m/1019549.1603770457@sss.pgh.pa.us Discussion: https://postgr.es/m/AM6PR02MB5287A0ADD936C1FA80973E72AB190@AM6PR02MB5287.eurprd02.prod.outlook.com
* Optimize a few list_delete_ptr callsDavid Rowley2020-10-221-2/+7
| | | | | | | | | | | | | | | | | | | | | There is a handful of places where we called list_delete_ptr() to remove some element from a List. In many of these places we know, or with very little additional effort know the index of the ListCell that we need to remove. Here we change all of those places to instead either use one of; list_delete_nth_cell(), foreach_delete_current() or list_delete_last(). Each of these saves from having to iterate over the list to search for the element to remove by its pointer value. There are some small performance gains to be had by doing this, but in the general case, none of these lists are likely to be very large, so the lookup was probably never that expensive anyway. However, some of the calls are in fairly hot code paths, e.g process_equivalence(). So any small gains there are useful. Author: Zhijie Hou and David Rowley Discussion: https://postgr.es/m/b3517353ec7c4f87aa560678fbb1034b@G08CNEXMBPEKD05.g08.fujitsu.local
* Build EC members for child join rels in the right memory context.Tom Lane2020-10-061-0/+13
| | | | | | | | | | | | | | | | | This patch prevents crashes or wrong plans when partition-wise joins are considered during GEQO planning, as a consequence of the EquivalenceClass data structures becoming corrupt after a GEQO context reset. A remaining problem is that successive GEQO cycles will make multiple copies of the required EC members, since add_child_join_rel_equivalences has no idea that such members might exist already. For now we'll just live with that. The lack of field complaints of crashes suggests that this is a mighty little-used situation. Back-patch to v12 where this code was introduced. Discussion: https://postgr.es/m/1683100.1601860653@sss.pgh.pa.us
* Fix two latent(?) bugs in equivclass.c.Tom Lane2020-10-051-8/+8
| | | | | | | | | | | | | | | | | | | | | | | | | | | get_eclass_for_sort_expr() computes expr_relids and nullable_relids early on, even though they won't be needed unless we make a new EquivalenceClass, which we often don't. Aside from the probably-minor inefficiency, there's a memory management problem: these bitmapsets will be built in the caller's context, leading to dangling pointers if that is shorter-lived than root->planner_cxt. This would be a live bug if get_eclass_for_sort_expr() could be called with create_it = true during GEQO join planning. So far as I can find, the core code never does that, but it's hard to be sure that no extensions do, especially since the comments make it clear that that's supposed to be a supported case. Fix by not computing these values until we've switched into planner_cxt to build the new EquivalenceClass. generate_join_implied_equalities() uses inner_rel->relids to look up relevant eclasses, but it ought to be using nominal_inner_relids. This is presently harmless because a child RelOptInfo will always have exactly the same eclass_indexes as its topmost parent; but that might not be true forever, and anyway it makes the code confusing. The first of these is old (introduced by me in f3b3b8d5b), so back-patch to all supported branches. The second only dates to v13, but we might as well back-patch it to keep the code looking similar across branches. Discussion: https://postgr.es/m/1508010.1601832581@sss.pgh.pa.us
* Suppress unnecessary RelabelType nodes in yet more cases.Tom Lane2020-08-191-26/+17
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | Commit a477bfc1d fixed eval_const_expressions() to ensure that it didn't generate unnecessary RelabelType nodes, but I failed to notice that some other places in the planner had the same issue. Really noplace in the planner should be using plain makeRelabelType(), for fear of generating expressions that should be equal() to semantically equivalent trees, but aren't. An example is that because canonicalize_ec_expression() failed to be careful about this, we could end up with an equivalence class containing both a plain Const, and a Const-with-RelabelType representing exactly the same value. So far as I can tell this led to no visible misbehavior, but we did waste a bunch of cycles generating and evaluating "Const = Const-with-RelabelType" to prove such entries are redundant. Hence, move the support function added by a477bfc1d to where it can be more generally useful, and use it in the places where planner code previously used makeRelabelType. Back-patch to v12, like the previous patch. While I have no concrete evidence of any real misbehavior here, it's certainly possible that I overlooked a case where equivalent expressions that aren't equal() could cause a user-visible problem. In any case carrying extra RelabelType nodes through planning to execution isn't very desirable. Discussion: https://postgr.es/m/1311836.1597781384@sss.pgh.pa.us
* Consider Incremental Sort paths at additional placesTomas Vondra2020-04-071-0/+28
| | | | | | | | | | | | | | | | | | | Commit d2d8a229bc introduced Incremental Sort, but it was considered only in create_ordered_paths() as an alternative to regular Sort. There are many other places that require sorted input and might benefit from considering Incremental Sort too. This patch modifies a number of those places, but not all. The concern is that just adding Incremental Sort to any place that already adds Sort may increase the number of paths considered, negatively affecting planning time, without any benefit. So we've taken a more conservative approach, based on analysis of which places do affect a set of queries that did seem practical. This means some less common queries may not benefit from Incremental Sort yet. Author: Tomas Vondra Reviewed-by: James Coleman Discussion: https://postgr.es/m/CAPpHfds1waRZ=NOmueYq0sx1ZSCnt+5QJvizT8ndT2=etZEeAQ@mail.gmail.com
* Update copyrights for 2020Bruce Momjian2020-01-011-1/+1
| | | | Backpatch-through: update all files in master, backpatch legal files through 9.4
* Generate EquivalenceClass members for partitionwise child join rels.Tom Lane2019-11-051-13/+148
| | | | | | | | | | | | | | | | | | | | | | | | | | | Commit d25ea0127 got rid of what I thought were entirely unnecessary derived child expressions in EquivalenceClasses for EC members that mention multiple baserels. But it turns out that some of the child expressions that code created are necessary for partitionwise joins, else we fail to find matching pathkeys for Sort nodes. (This happens only for certain shapes of the resulting plan; it may be that partitionwise aggregation is also necessary to show the failure, though I'm not sure of that.) Reverting that commit entirely would be quite painful performance-wise for large partition sets. So instead, add code that explicitly generates child expressions that match only partitionwise child join rels we have actually generated. Per report from Justin Pryzby. (Amit Langote noticed the problem earlier, though it's not clear if he recognized then that it could result in a planner error, not merely failure to exploit partitionwise join, in the code as-committed.) Back-patch to v12 where commit d25ea0127 came in. Amit Langote, with lots of kibitzing from me Discussion: https://postgr.es/m/CA+HiwqG2WVUGmLJqtR0tPFhniO=H=9qQ+Z3L_ZC+Y3-EVQHFGg@mail.gmail.com Discussion: https://postgr.es/m/20191011143703.GN10470@telsasoft.com
* Adjust overly strict AssertDavid Rowley2019-07-221-1/+2
| | | | | | | | | | | | | 3373c7155 changed how we determine EquivalenceClasses for relations and added an Assert to ensure all relations mentioned in each EC's ec_relids was a RELOPT_BASEREL. However, the join removal code may remove a LEFT JOIN and since it does not clean up EC members belonging to the removed relations it can leave RELOPT_DEADREL rels in ec_relids. Fix this by adjusting the Assert to allow RELOPT_DEADREL rels too. Reported-by: sqlsmith via Andreas Seltenreich Discussion: https://postgr.es/m/87y30r8sls.fsf@ansel.ydns.eu
* Speed up finding EquivalenceClasses for a given set of relsDavid Rowley2019-07-211-79/+273
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Previously in order to determine which ECs a relation had members in, we had to loop over all ECs stored in PlannerInfo's eq_classes and check if ec_relids mentioned the relation. For the most part, this was fine, as generally, unless queries were fairly complex, the overhead of performing the lookup would have not been that significant. However, when queries contained large numbers of joins and ECs, the overhead to find the set of classes matching a given set of relations could become a significant portion of the overall planning effort. Here we allow a much more efficient method to access the ECs which match a given relation or set of relations. A new Bitmapset field in RelOptInfo now exists to store the indexes into PlannerInfo's eq_classes list which each relation is mentioned in. This allows very fast lookups to find all ECs belonging to a single relation. When we need to lookup ECs belonging to a given pair of relations, we can simply bitwise-AND the Bitmapsets from each relation and use the result to perform the lookup. We also take the opportunity to write a new implementation of generate_join_implied_equalities which makes use of the new indexes. generate_join_implied_equalities_for_ecs must remain as is as it can be given a custom list of ECs, which we can't easily determine the indexes of. This was originally intended to fix the performance penalty of looking up foreign keys matching a join condition which was introduced by 100340e2d. However, we're speeding up much more than just that here. Author: David Rowley, Tom Lane Reviewed-by: Tom Lane, Tomas Vondra Discussion: https://postgr.es/m/6970.1545327857@sss.pgh.pa.us
* Represent Lists as expansible arrays, not chains of cons-cells.Tom Lane2019-07-151-23/+15
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Originally, Postgres Lists were a more or less exact reimplementation of Lisp lists, which consist of chains of separately-allocated cons cells, each having a value and a next-cell link. We'd hacked that once before (commit d0b4399d8) to add a separate List header, but the data was still in cons cells. That makes some operations -- notably list_nth() -- O(N), and it's bulky because of the next-cell pointers and per-cell palloc overhead, and it's very cache-unfriendly if the cons cells end up scattered around rather than being adjacent. In this rewrite, we still have List headers, but the data is in a resizable array of values, with no next-cell links. Now we need at most two palloc's per List, and often only one, since we can allocate some values in the same palloc call as the List header. (Of course, extending an existing List may require repalloc's to enlarge the array. But this involves just O(log N) allocations not O(N).) Of course this is not without downsides. The key difficulty is that addition or deletion of a list entry may now cause other entries to move, which it did not before. For example, that breaks foreach() and sister macros, which historically used a pointer to the current cons-cell as loop state. We can repair those macros transparently by making their actual loop state be an integer list index; the exposed "ListCell *" pointer is no longer state carried across loop iterations, but is just a derived value. (In practice, modern compilers can optimize things back to having just one loop state value, at least for simple cases with inline loop bodies.) In principle, this is a semantics change for cases where the loop body inserts or deletes list entries ahead of the current loop index; but I found no such cases in the Postgres code. The change is not at all transparent for code that doesn't use foreach() but chases lists "by hand" using lnext(). The largest share of such code in the backend is in loops that were maintaining "prev" and "next" variables in addition to the current-cell pointer, in order to delete list cells efficiently using list_delete_cell(). However, we no longer need a previous-cell pointer to delete a list cell efficiently. Keeping a next-cell pointer doesn't work, as explained above, but we can improve matters by changing such code to use a regular foreach() loop and then using the new macro foreach_delete_current() to delete the current cell. (This macro knows how to update the associated foreach loop's state so that no cells will be missed in the traversal.) There remains a nontrivial risk of code assuming that a ListCell * pointer will remain good over an operation that could now move the list contents. To help catch such errors, list.c can be compiled with a new define symbol DEBUG_LIST_MEMORY_USAGE that forcibly moves list contents whenever that could possibly happen. This makes list operations significantly more expensive so it's not normally turned on (though it is on by default if USE_VALGRIND is on). There are two notable API differences from the previous code: * lnext() now requires the List's header pointer in addition to the current cell's address. * list_delete_cell() no longer requires a previous-cell argument. These changes are somewhat unfortunate, but on the other hand code using either function needs inspection to see if it is assuming anything it shouldn't, so it's not all bad. Programmers should be aware of these significant performance changes: * list_nth() and related functions are now O(1); so there's no major access-speed difference between a list and an array. * Inserting or deleting a list element now takes time proportional to the distance to the end of the list, due to moving the array elements. (However, it typically *doesn't* require palloc or pfree, so except in long lists it's probably still faster than before.) Notably, lcons() used to be about the same cost as lappend(), but that's no longer true if the list is long. Code that uses lcons() and list_delete_first() to maintain a stack might usefully be rewritten to push and pop at the end of the list rather than the beginning. * There are now list_insert_nth...() and list_delete_nth...() functions that add or remove a list cell identified by index. These have the data-movement penalty explained above, but there's no search penalty. * list_concat() and variants now copy the second list's data into storage belonging to the first list, so there is no longer any sharing of cells between the input lists. The second argument is now declared "const List *" to reflect that it isn't changed. This patch just does the minimum needed to get the new implementation in place and fix bugs exposed by the regression tests. As suggested by the foregoing, there's a fair amount of followup work remaining to do. Also, the ENABLE_LIST_COMPAT macros are finally removed in this commit. Code using those should have been gone a dozen years ago. Patch by me; thanks to David Rowley, Jesper Pedersen, and others for review. Discussion: https://postgr.es/m/11587.1550975080@sss.pgh.pa.us
* Fix many typos and inconsistenciesMichael Paquier2019-07-011-5/+4
| | | | | Author: Alexander Lakhin Discussion: https://postgr.es/m/af27d1b3-a128-9d62-46e0-88f424397f44@gmail.com
* Avoid combinatorial explosion in add_child_rel_equivalences().Tom Lane2019-06-131-13/+36
| | | | | | | | | | | | | | | | | | | | | | | | If an EquivalenceClass member expression includes variables from multiple appendrels, then instead of producing one substituted expression per child relation as intended, we'd create additional child expressions for combinations of children of different appendrels. This happened because the child expressions generated while considering the first appendrel were taken as sources during substitution of the second appendrel, and so on. The extra expressions are useless, and are harmless unless there are too many of them --- but if you have several appendrels with a thousand or so members each, it gets bad fast. To fix, consider only original (non-em_is_child) EC members as candidates to be expanded. This requires the ability to substitute directly from a top parent relation's Vars to those of an indirect descendant relation, but we already have that in adjust_appendrel_attrs_multilevel(). Per bug #15847 from Feike Steenbergen. This is a longstanding misbehavior, but it's only worth worrying about when there are more appendrel children than we've historically considered wise to use. So I'm not going to take the risk of back-patching this. Discussion: https://postgr.es/m/15847-ea3734094bf8ae61@postgresql.org
* Phase 2 pgindent run for v12.Tom Lane2019-05-221-23/+23
| | | | | | | | | Switch to 2.1 version of pg_bsd_indent. This formats multiline function declarations "correctly", that is with additional lines of parameter declarations indented to match where the first line's left parenthesis is. Discussion: https://postgr.es/m/CAEepm=0P3FeTXRcU5B2W3jv3PgRVZ-kGUXLGfd42FFhUROO3ug@mail.gmail.com
* Speed up match_eclasses_to_foreign_key_col() when there are many ECs.Tom Lane2019-02-201-0/+8
| | | | | | | | | | | | | Check ec_relids before bothering to iterate through the EC members. On a perhaps extreme, but still real-world, query in which match_eclasses_to_foreign_key_col() accounts for the bulk of the planner's runtime, this saves nearly 40% of the runtime. It's a bit of a stopgap fix, but it's simple enough to be back-patched to 9.6 where this code came in; so let's do that. David Rowley Discussion: https://postgr.es/m/6970.1545327857@sss.pgh.pa.us
* Refactor the representation of indexable clauses in IndexPaths.Tom Lane2019-02-091-0/+37
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | In place of three separate but interrelated lists (indexclauses, indexquals, and indexqualcols), an IndexPath now has one list "indexclauses" of IndexClause nodes. This holds basically the same information as before, but in a more useful format: in particular, there is now a clear connection between an indexclause (an original restriction clause from WHERE or JOIN/ON) and the indexquals (directly usable index conditions) derived from it. We also change the ground rules a bit by mandating that clause commutation, if needed, be done up-front so that what is stored in the indexquals list is always directly usable as an index condition. This gets rid of repeated re-determination of which side of the clause is the indexkey during costing and plan generation, as well as repeated lookups of the commutator operator. To minimize the added up-front cost, the typical case of commuting a plain OpExpr is handled by a new special-purpose function commute_restrictinfo(). For RowCompareExprs, generating the new clause properly commuted to begin with is not really any more complex than before, it's just different --- and we can save doing that work twice, as the pretty-klugy original implementation did. Tracking the connection between original and derived clauses lets us also track explicitly whether the derived clauses are an exact or lossy translation of the original. This provides a cheap solution to getting rid of unnecessary rechecks of boolean index clauses, which previously seemed like it'd be more expensive than it was worth. Another pleasant (IMO) side-effect is that EXPLAIN now always shows index clauses with the indexkey on the left; this seems less confusing. This commit leaves expand_indexqual_conditions() and some related functions in a slightly messy state. I didn't bother to change them any more than minimally necessary to work with the new data structure, because all that code is going to be refactored out of existence in a follow-on patch. Discussion: https://postgr.es/m/22182.1549124950@sss.pgh.pa.us
* Refactor planner's header files.Tom Lane2019-01-291-1/+1
| | | | | | | | | | | | | | | | | | | | | | | | Create a new header optimizer/optimizer.h, which exposes just the planner functions that can be used "at arm's length", without need to access Paths or the other planner-internal data structures defined in nodes/relation.h. This is intended to provide the whole planner API seen by most of the rest of the system; although FDWs still need to use additional stuff, and more thought is also needed about just what selfuncs.c should rely on. The main point of doing this now is to limit the amount of new #include baggage that will be needed by "planner support functions", which I expect to introduce later, and which will be in relevant datatype modules rather than anywhere near the planner. This commit just moves relevant declarations into optimizer.h from other header files (a couple of which go away because everything got moved), and adjusts #include lists to match. There's further cleanup that could be done if we want to decide that some stuff being exposed by optimizer.h doesn't belong in the planner at all, but I'll leave that for another day. Discussion: https://postgr.es/m/11460.1548706639@sss.pgh.pa.us
* Move inheritance expansion code into its own fileAlvaro Herrera2019-01-101-1/+1
| | | | | | | | | | | | | | | | | This commit moves expand_inherited_tables and underlings from optimizer/prep/prepunionc.c to optimizer/utils/inherit.c. Also, all of the AppendRelInfo-based expression manipulation routines are moved to optimizer/utils/appendinfo.c. No functional code changes. One exception is the introduction of make_append_rel_info, but that's still just moving around code. Also, stop including <limits.h> in prepunion.c, which no longer needs it since 3fc6e2d7f5b6. I (Álvaro) noticed this because Amit was copying that to inherit.c, which likewise doesn't need it. Author: Amit Langote Discussion: https://postgr.es/m/3be67028-a00a-502c-199a-da00eec8fb6e@lab.ntt.co.jp
* Update copyright for 2019Bruce Momjian2019-01-021-1/+1
| | | | Backpatch-through: certain files through 9.4
* Fix misprocessing of equivalence classes involving record_eq().Tom Lane2018-05-161-1/+2
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | canonicalize_ec_expression() is supposed to agree with coerce_type() as to whether a RelabelType should be inserted to make a subexpression be valid input for the operators of a given opclass. However, it did the wrong thing with named-composite-type inputs to record_eq(): it put in a RelabelType to RECORDOID, which the parser doesn't. In some cases this was harmless because all code paths involving a particular equivalence class did the same thing, but in other cases this would result in failing to recognize a composite-type expression as being a member of an equivalence class that it actually is a member of. The most obvious bad effect was to fail to recognize that an index on a composite column could provide the sort order needed for a mergejoin on that column, as reported by Teodor Sigaev. I think there might be other, subtler, cases that result in misoptimization. It also seems possible that an unwanted RelabelType would sometimes get into an emitted plan --- but because record_eq and friends don't examine the declared type of their input expressions, that would not create any visible problems. To fix, just treat RECORDOID as if it were a polymorphic type, which in some sense it is. We might want to consider formalizing that a bit more someday, but for the moment this seems to be the only place where an IsPolymorphicType() test ought to include RECORDOID as well. This has been broken for a long time, so back-patch to all supported branches. Discussion: https://postgr.es/m/a6b22369-e3bf-4d49-f59d-0c41d3551e81@sigaev.ru
* Update copyright for 2018Bruce Momjian2018-01-021-1/+1
| | | | Backpatch-through: certain files through 9.3
* Change TRUE/FALSE to true/falsePeter Eisentraut2017-11-081-5/+5
| | | | | | | | | | | | | | The lower case spellings are C and C++ standard and are used in most parts of the PostgreSQL sources. The upper case spellings are only used in some files/modules. So standardize on the standard spellings. The APIs for ICU, Perl, and Windows define their own TRUE and FALSE, so those are left as is when using those APIs. In code comments, we use the lower-case spelling for the C concepts and keep the upper-case spelling for the SQL concepts. Reviewed-by: Michael Paquier <michael.paquier@gmail.com>
* Reduce "X = X" to "X IS NOT NULL", if it's easy to do so.Tom Lane2017-10-081-14/+52
| | | | | | | | | | | | | | | | | | | | | If the operator is a strict btree equality operator, and X isn't volatile, then the clause must yield true for any non-null value of X, or null if X is null. At top level of a WHERE clause, we can ignore the distinction between false and null results, so it's valid to simplify the clause to "X IS NOT NULL". This is a useful improvement mainly because we'll get a far better selectivity estimate in most cases. Because such cases seldom arise in well-written queries, it is unappetizing to expend a lot of planner cycles looking for them ... but it turns out that there's a place we can shoehorn this in practically for free, because equivclass.c already has to detect and reject candidate equivalences of the form X = X. That doesn't catch every place that it would be valid to simplify to X IS NOT NULL, but it catches the typical case. Working harder doesn't seem justified. Patch by me, reviewed by Petr Jelinek Discussion: https://postgr.es/m/CAMjNa7cC4X9YR-vAJS-jSYCajhRDvJQnN7m2sLH1wLh-_Z2bsw@mail.gmail.com
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