From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | pgsql-committers(at)postgresql(dot)org |
Subject: | pgsql: Remove dubious micro-optimization in ckpt_buforder_comparator(). |
Date: | 2018-01-10 20:51:03 |
Message-ID: | E1eZNKx-0000Fw-0S@gemulon.postgresql.org |
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Lists: | pgsql-committers |
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
Branch
------
REL9_6_STABLE
Details
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https://git.postgresql.org/pg/commitdiff/c24a908d04806c34c3e5dda42d39bb5ca1fc5ffb
Modified Files
--------------
src/backend/storage/buffer/bufmgr.c | 12 +++++++-----
1 file changed, 7 insertions(+), 5 deletions(-)
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