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Remove dubious micro-optimization in ckpt_buforder_comparator(). 

It seems incorrect to assume that the list of CkptSortItems can never
contain duplicate page numbers: concurrent activity could result in some
page getting dropped from a low-numbered buffer and later loaded into a
high-numbered buffer while BufferSync is scanning the buffer pool.
If that happened, the comparator would give self-inconsistent results,
potentially confusing qsort().  Saving one comparison step is not worth
possibly getting the sort wrong.

So far as I can tell, nothing would actually go wrong given our current
implementation of qsort().  It might get a bit slower than expected
if there were a large number of duplicates of one value, but that's
surely a probability-epsilon case.  Still, the comment is wrong,
and if we ever switched to another sort implementation it might be
less forgiving.

In passing, avoid casting away const-ness of the argument pointers;
I've not seen any compiler complaints from that, but it seems likely
that some compilers would not like it.

Back-patch to 9.6 where this code came in, just in case I've underestimated
the possible consequences.

Discussion: https://postgr.es/m/18437.1515607610@sss.pgh.pa.us
This commit is contained in:
Tom Lane 2018-01-10 15:50:54 -05:00
parent 2fd58096f0
commit 3afd75eaac
1 changed files with 7 additions and 5 deletions
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12
src/backend/storage/buffer/bufmgr.c
View File

@ -4064,8 +4064,8 @@ local_buffer_write_error_callback(void *arg)
static int
rnode_comparator(const void *p1, const void *p2)
{
RelFileNode n1 = *(RelFileNode *) p1;
RelFileNode n2 = *(RelFileNode *) p2;
RelFileNode n1 = *(const RelFileNode *) p1;
RelFileNode n2 = *(const RelFileNode *) p2;
if (n1.relNode < n2.relNode)
return -1;
@ -4174,8 +4174,8 @@ buffertag_comparator(const void *a, const void *b)
static int
ckpt_buforder_comparator(const void *pa, const void *pb)
{
const CkptSortItem *a = (CkptSortItem *) pa;
const CkptSortItem *b = (CkptSortItem *) pb;
const CkptSortItem *a = (const CkptSortItem *) pa;
const CkptSortItem *b = (const CkptSortItem *) pb;
/* compare tablespace */
if (a->tsId < b->tsId)
@ -4195,8 +4195,10 @@ ckpt_buforder_comparator(const void *pa, const void *pb)
/* compare block number */
else if (a->blockNum < b->blockNum)
return -1;
else /* should not be the same block ... */
else if (a->blockNum > b->blockNum)
return 1;
/* equal page IDs are unlikely, but not impossible */
return 0;
}
/*