From fe939d950e362094c14e97f4e91c063374588534 Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Fri, 21 Aug 2015 20:32:11 -0400
Subject: [PATCH] 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
---
src/backend/executor/spi.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/src/backend/executor/spi.c b/src/backend/executor/spi.c
index cfa4a24686f..649040e8fd7 100644
--- a/src/backend/executor/spi.c
+++ b/src/backend/executor/spi.c
@@ -1769,7 +1769,8 @@ 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));
--
GitLab