diff options
Diffstat (limited to 'doc')
| -rw-r--r-- | doc/src/sgml/indexam.sgml | 20 |
1 files changed, 18 insertions, 2 deletions
diff --git a/doc/src/sgml/indexam.sgml b/doc/src/sgml/indexam.sgml index 65da721de47..1b42ff0231a 100644 --- a/doc/src/sgml/indexam.sgml +++ b/doc/src/sgml/indexam.sgml @@ -1,4 +1,4 @@ -<!-- $PostgreSQL: pgsql/doc/src/sgml/indexam.sgml,v 2.24 2008/04/10 22:25:25 tgl Exp $ --> +<!-- $PostgreSQL: pgsql/doc/src/sgml/indexam.sgml,v 2.25 2008/04/13 19:18:13 tgl Exp $ --> <chapter id="indexam"> <title>Index Access Method Interface Definition</title> @@ -315,7 +315,15 @@ amgettuple (IndexScanDesc scan, TID is stored into the <literal>scan</> structure. Note that <quote>success</> means only that the index contains an entry that matches the scan keys, not that the tuple necessarily still exists in the heap or - will pass the caller's snapshot test. + will pass the caller's snapshot test. On success, <function>amgettuple</> + must also set <literal>scan->xs_recheck</> to TRUE or FALSE. + FALSE means it is certain that the index entry matches the scan keys. + TRUE means this is not certain, and the conditions represented by the + scan keys must be rechecked against the heap tuple after fetching it. + This provision supports <quote>lossy</> index operators. + Note that rechecking will extend only to the scan conditions; a partial + index predicate (if any) is never rechecked by <function>amgettuple</> + callers. </para> <para> @@ -327,6 +335,14 @@ amgetbitmap (IndexScanDesc scan, Fetch all tuples in the given scan and add them to the caller-supplied TIDBitmap (that is, OR the set of tuple IDs into whatever set is already in the bitmap). The number of tuples fetched is returned. + While inserting tuple IDs into the bitmap, <function>amgetbitmap</> can + indicate that rechecking of the scan conditions is required for specific + tuple IDs. This is analogous to the <literal>xs_recheck</> output parameter + of <function>amgettuple</>. Note: in the current implementation, support + for this feature is conflated with support for lossy storage of the bitmap + itself, and therefore callers recheck both the scan conditions and the + partial index predicate (if any) for recheckable tuples. That might not + always be true, however. <function>amgetbitmap</> and <function>amgettuple</> cannot be used in the same index scan; there are other restrictions too when using <function>amgetbitmap</>, as explained |