From c119c5bd49baa424480bd9e8f9dda69a09f5a572 Mon Sep 17 00:00:00 2001 From: Neil Conway Date: Wed, 15 Jun 2005 07:27:44 +0000 Subject: Change the implementation of hash join to attempt to avoid unnecessary work if either of the join relations are empty. The logic is: (1) if the inner relation's startup cost is less than the outer relation's startup cost and this is not an outer join, read a single tuple from the inner relation via ExecHash() - if NULL, we're done (2) read a single tuple from the outer relation - if NULL, we're done (3) build the hash table on the inner relation - if hash table is empty and this is not an outer join, we're done (4) otherwise, do hash join as usual The implementation uses the new MultiExecProcNode API, per a suggestion from Tom: invoking ExecHash() now produces the first tuple from the Hash node's child node, whereas MultiExecHash() builds the hash table. I had to put in a bit of a kludge to get the row count returned for EXPLAIN ANALYZE to be correct: since ExecHash() is invoked to return a tuple, and then MultiExecHash() is invoked, we would return one too many tuples to EXPLAIN ANALYZE. I hacked around this by just manually detecting this situation and subtracting 1 from the EXPLAIN ANALYZE row count. --- src/include/nodes/execnodes.h | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) (limited to 'src/include/nodes/execnodes.h') diff --git a/src/include/nodes/execnodes.h b/src/include/nodes/execnodes.h index b92eb1722f..19f264119c 100644 --- a/src/include/nodes/execnodes.h +++ b/src/include/nodes/execnodes.h @@ -7,7 +7,7 @@ * Portions Copyright (c) 1996-2005, PostgreSQL Global Development Group * Portions Copyright (c) 1994, Regents of the University of California * - * $PostgreSQL: pgsql/src/include/nodes/execnodes.h,v 1.133 2005/05/14 21:29:23 tgl Exp $ + * $PostgreSQL: pgsql/src/include/nodes/execnodes.h,v 1.134 2005/06/15 07:27:44 neilc Exp $ * *------------------------------------------------------------------------- */ @@ -1218,6 +1218,7 @@ typedef struct HashState HashJoinTable hashtable; /* hash table for the hashjoin */ List *hashkeys; /* list of ExprState nodes */ /* hashkeys is same as parent's hj_InnerHashKeys */ + TupleTableSlot *firstTuple; /* tuple produced by ExecHash() */ } HashState; /* ---------------- -- cgit v1.2.1