From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Sat, 22 Aug 2015 00:32:11 +0000 (-0400)
Subject: Avoid O(N^2) behavior when enlarging SPI tuple table in spi_printtup().
X-Git-Tag: REL9_0_23~50
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=7c57449fc5855361d3a69374192c58fb68ce3e67;p=thirdparty%2Fpostgresql.git

Avoid O(N^2) behavior when enlarging SPI tuple table in spi_printtup().

For no obvious reason, spi_printtup() was coded to enlarge the tuple
pointer table by just 256 slots at a time, rather than doubling the size at
each reallocation, as is our usual habit.  For very large SPI results, this
makes for O(N^2) time spent in repalloc(), which of course soon comes to
dominate the runtime.  Use the standard doubling approach instead.

This is a longstanding performance bug, so back-patch to all active
branches.

Neil Conway
---

diff --git a/src/backend/executor/spi.c b/src/backend/executor/spi.c
index 7de785d7257..a9b544ab0eb 100644
--- a/src/backend/executor/spi.c
+++ b/src/backend/executor/spi.c
@@ -1622,7 +1622,8 @@ spi_printtup(TupleTableSlot *slot, DestReceiver *self)
 
 	if (tuptable->free == 0)
 	{
-		tuptable->free = 256;
+		/* Double the size of the pointer array */
+		tuptable->free = tuptable->alloced;
 		tuptable->alloced += tuptable->free;
 		tuptable->vals = (HeapTuple *) repalloc(tuptable->vals,
 									  tuptable->alloced * sizeof(HeapTuple));