From: Tom Lane Date: Sat, 26 May 2012 23:09:52 +0000 (-0400) Subject: Prevent synchronized scanning when systable_beginscan chooses a heapscan. X-Git-Tag: REL9_2_BETA2~26 X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=532fe28dade9291b7a7f7de3920a4d997bbd86a1;p=postgresql Prevent synchronized scanning when systable_beginscan chooses a heapscan. The only interesting-for-performance case wherein we force heapscan here is when we're rebuilding the relcache init file, and the only such case that is likely to be examining a catalog big enough to be syncscanned is RelationBuildTupleDesc. But the early-exit optimization in that code gets broken if we start the scan at a random place within the catalog, so that allowing syncscan is actually a big deoptimization if pg_attribute is large (at least for the normal case where the rows for core system catalogs have never been changed since initdb). Hence, prevent syncscan here. Per my testing pursuant to complaints from Jeff Frost and Greg Sabino Mullane, though neither of them seem to have actually hit this specific problem. Back-patch to 8.3, where syncscan was introduced. --- diff --git a/src/backend/access/index/genam.c b/src/backend/access/index/genam.c index e94c3957f2..d54b669bf3 100644 --- a/src/backend/access/index/genam.c +++ b/src/backend/access/index/genam.c @@ -293,7 +293,16 @@ systable_beginscan(Relation heapRelation, } else { - sysscan->scan = heap_beginscan(heapRelation, snapshot, nkeys, key); + /* + * We disallow synchronized scans when forced to use a heapscan on a + * catalog. In most cases the desired rows are near the front, so + * that the unpredictable start point of a syncscan is a serious + * disadvantage; and there are no compensating advantages, because + * it's unlikely that such scans will occur in parallel. + */ + sysscan->scan = heap_beginscan_strat(heapRelation, snapshot, + nkeys, key, + true, false); sysscan->iscan = NULL; }