diff options
Diffstat (limited to 'doc/src/sgml/indexam.sgml')
| -rw-r--r-- | doc/src/sgml/indexam.sgml | 12 |
1 files changed, 5 insertions, 7 deletions
diff --git a/doc/src/sgml/indexam.sgml b/doc/src/sgml/indexam.sgml index 51e70e9200..241064a40f 100644 --- a/doc/src/sgml/indexam.sgml +++ b/doc/src/sgml/indexam.sgml @@ -105,14 +105,15 @@ where no indexable restriction clause is given for the first index column. When <structfield>amcanmulticol</structfield> is false, <structfield>amoptionalkey</structfield> essentially says whether the - access method allows full-index scans without any restriction clause. + access method supports full-index scans without any restriction clause. Access methods that support multiple index columns <emphasis>must</> support scans that omit restrictions on any or all of the columns after the first; however they are permitted to require some restriction to appear for the first index column, and this is signaled by setting <structfield>amoptionalkey</structfield> false. - <structfield>amindexnulls</structfield> asserts that index entries are - created for NULL key values. Since most indexable operators are + One reason that an index AM might set + <structfield>amoptionalkey</structfield> false is if it doesn't index + NULLs. Since most indexable operators are strict and hence cannot return TRUE for NULL inputs, it is at first sight attractive to not store index entries for null values: they could never be returned by an index scan anyway. However, this @@ -129,10 +130,7 @@ used to scan for rows with <literal>a = 4</literal>, which is wrong if the index omits rows where <literal>b</> is null. It is, however, OK to omit rows where the first indexed column is null. - Thus, <structfield>amindexnulls</structfield> should be set true only if the - index access method indexes all rows, including arbitrary combinations of - null values. An index access method that sets - <structfield>amindexnulls</structfield> may also set + An index access method that does index nulls may also set <structfield>amsearchnulls</structfield>, indicating that it supports <literal>IS NULL</> and <literal>IS NOT NULL</> clauses as search conditions. |