postgres.git / summary / log / commit / refs

commit    532fe28dade9291b7a7f7de3920a4d997bbd86a1
Author:   Tom Lane <tgl@sss.pgh.pa.us>
Date:     Sat May 26 23:09:52 2012 +0000
First released in: 9.2.0, 8.3.19, 8.4.12, 9.0.8, 9.1.4

    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.


 src/backend/access/index/genam.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/src/backend/access/index/genam.c b/src/backend/access/index/genam.c
index e94c3957f26..d54b669bf35 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;
 	}
 


[parent: d3b97d1488ba]