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From: Dan Ports <drkp@csail.mit.edu>
To: pgsql-hackers@postgresql.org, Kevin Grittner <Kevin.Grittner@wicourts.gov>
Subject: Repeated PredicateLockRelation calls during seqscan
Date: Wed, 22 Jun 2011 00:58:50 -0400
Message-ID: <20110622045850.GN83336@csail.mit.edu> (raw)

I was looking at ExecSeqScan today and noticed that it invokes
PredicateLockRelation each time it's called, i.e. for each tuple
returned. Any reason we shouldn't skip that call if
rs_relpredicatelocked is already set, as in the attached patch?

That would save us a bit of overhead, since checking that flag is
cheaper than doing a hash lookup in the local predicate lock table
before bailing out.

Dan

-- 
Dan R. K. Ports              MIT CSAIL                http://drkp.net/

Attachments:

predlock-seqscan.patch (text/x-diff, 823B) download
diff --git a/src/backend/executor/nodeSeqscan.c b/src/backend/executor/nodeSeqscan.c
index f356874..32a8f56 100644
--- a/src/backend/executor/nodeSeqscan.c
+++ b/src/backend/executor/nodeSeqscan.c
@@ -113,9 +113,13 @@ SeqRecheck(SeqScanState *node, TupleTableSlot *slot)
 TupleTableSlot *
 ExecSeqScan(SeqScanState *node)
 {
-	PredicateLockRelation(node->ss_currentRelation,
-						  node->ss_currentScanDesc->rs_snapshot);
-	node->ss_currentScanDesc->rs_relpredicatelocked = true;
+	if (!node->ss_currentScanDesc->rs_relpredicatelocked)
+	{
+		PredicateLockRelation(node->ss_currentRelation,
+							  node->ss_currentScanDesc->rs_snapshot);
+		node->ss_currentScanDesc->rs_relpredicatelocked = true;		
+	}
+	
 	return ExecScan((ScanState *) node,
 					(ExecScanAccessMtd) SeqNext,
 					(ExecScanRecheckMtd) SeqRecheck);

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  To: pgsql-hackers@postgresql.org
  Cc: drkp@csail.mit.edu, Kevin.Grittner@wicourts.gov
  Subject: Re: Repeated PredicateLockRelation calls during seqscan
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