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Commit3afd75e

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Remove dubious micro-optimization in ckpt_buforder_comparator().
It seems incorrect to assume that the list of CkptSortItems can nevercontain duplicate page numbers: concurrent activity could result in somepage getting dropped from a low-numbered buffer and later loaded into ahigh-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 worthpossibly getting the sort wrong.So far as I can tell, nothing would actually go wrong given our currentimplementation of qsort(). It might get a bit slower than expectedif there were a large number of duplicates of one value, but that'ssurely a probability-epsilon case. Still, the comment is wrong,and if we ever switched to another sort implementation it might beless forgiving.In passing, avoid casting away const-ness of the argument pointers;I've not seen any compiler complaints from that, but it seems likelythat some compilers would not like it.Back-patch to 9.6 where this code came in, just in case I've underestimatedthe possible consequences.Discussion:https://postgr.es/m/18437.1515607610@sss.pgh.pa.us
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‎src/backend/storage/buffer/bufmgr.c

Lines changed: 7 additions & 5 deletions
Original file line numberDiff line numberDiff line change
@@ -4064,8 +4064,8 @@ local_buffer_write_error_callback(void *arg)
40644064
staticint
40654065
rnode_comparator(constvoid*p1,constvoid*p2)
40664066
{
4067-
RelFileNoden1=*(RelFileNode*)p1;
4068-
RelFileNoden2=*(RelFileNode*)p2;
4067+
RelFileNoden1=*(constRelFileNode*)p1;
4068+
RelFileNoden2=*(constRelFileNode*)p2;
40694069

40704070
if (n1.relNode<n2.relNode)
40714071
return-1;
@@ -4174,8 +4174,8 @@ buffertag_comparator(const void *a, const void *b)
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staticint
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ckpt_buforder_comparator(constvoid*pa,constvoid*pb)
41764176
{
4177-
constCkptSortItem*a= (CkptSortItem*)pa;
4178-
constCkptSortItem*b= (CkptSortItem*)pb;
4177+
constCkptSortItem*a= (constCkptSortItem*)pa;
4178+
constCkptSortItem*b= (constCkptSortItem*)pb;
41794179

41804180
/* compare tablespace */
41814181
if (a->tsId<b->tsId)
@@ -4195,8 +4195,10 @@ ckpt_buforder_comparator(const void *pa, const void *pb)
41954195
/* compare block number */
41964196
elseif (a->blockNum<b->blockNum)
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return-1;
4198-
else/* should not be the same block ... */
4198+
elseif (a->blockNum>b->blockNum)
41994199
return1;
4200+
/* equal page IDs are unlikely, but not impossible */
4201+
return0;
42004202
}
42014203

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/*

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