From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
---|---|
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-0000Fv-0H@gemulon.postgresql.org |
Views: | Raw Message | Whole Thread | Download mbox | Resend email |
Thread: | |
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
------
REL_10_STABLE
Details
-------
https://git.postgresql.org/pg/commitdiff/7eb0187a97674210515d167e022429c24d74d47c
Modified Files
--------------
src/backend/storage/buffer/bufmgr.c | 12 +++++++-----
1 file changed, 7 insertions(+), 5 deletions(-)
From | Date | Subject | |
---|---|---|---|
Next Message | Peter Eisentraut | 2018-01-10 21:05:47 | pgsql: Revert "Move portal pinning from PL/pgSQL to SPI" |
Previous Message | Robert Haas | 2018-01-10 16:24:28 | pgsql: Add missing "return" statement to accumulate_append_subpath. |