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Fix incorrect math in DetermineSafeOldestOffset.

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The old formula didn't have enough parentheses, so it would do the wrong
thing, and it used / rather than % to find a remainder.  The effect of
these oversights is that the stop point chosen by the logic introduced in
commit b69bf30b9bfacafc733a9ba77c9587cf54d06c0c might be rather
meaningless.

Thomas Munro, reviewed by Kevin Grittner, with a whitespace tweak by me.
This commit is contained in:
Robert Haas 2015-05-07 11:00:47 -04:00
parent 82ec7c95b7
commit 7be47c56af
1 changed files with 2 additions and 1 deletions
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3
src/backend/access/transam/multixact.c
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@ -2495,7 +2495,8 @@ DetermineSafeOldestOffset(MultiXactId oldestMXact)
*/
oldestOffset = find_multixact_start(oldestMXact);
/* move back to start of the corresponding segment */
oldestOffset -= oldestOffset / MULTIXACT_MEMBERS_PER_PAGE * SLRU_PAGES_PER_SEGMENT;
oldestOffset -= oldestOffset %
(MULTIXACT_MEMBERS_PER_PAGE * SLRU_PAGES_PER_SEGMENT);
LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);
/* always leave one segment before the wraparound point */