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 7de785d7257ce2a5b1ec78ccfd70b0ed2a82ed71..a9b544ab0eb96e58c9222976b3817abcce87b0e9 100644 (file)
--- 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));