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Track the oldest XID that can be safely looked up in CLOG.

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This provides infrastructure for looking up arbitrary, user-supplied
XIDs without a risk of scary-looking failures from within the clog
module.  Normally, the oldest XID that can be safely looked up in CLOG
is the same as the oldest XID that can reused without causing
wraparound, and the latter is already tracked.  However, while
truncation is in progress, the values are different, so we must
keep track of them separately.

Craig Ringer, reviewed by Simon Riggs and by me.

Discussion: http://postgr.es/m/CAMsr+YHQiWNEi0daCTboS40T+V5s_+dst3PYv_8v2wNVH+Xx4g@mail.gmail.com
This commit is contained in:
Robert Haas
2017-03-23 14:08:23 -04:00
parent 50c956add8
commit ea42cc18c3
11 changed files with 102 additions and 18 deletions
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4
doc/src/sgml/monitoring.sgml
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@ -1017,6 +1017,10 @@ postgres 27093 0.0 0.0 30096 2752 ? Ss 11:34 0:00 postgres: ser
<entry><literal>OldSnapshotTimeMapLock</></entry>
<entry>Waiting to read or update old snapshot control information.</entry>
</row>
<row>
<entry><literal>CLogTruncationLock</></entry>
<entry>Waiting to truncate the transaction log or waiting for transaction log truncation to finish.</entry>
</row>
<row>
<entry><literal>clog</></entry>
<entry>Waiting for I/O on a clog (transaction status) buffer.</entry>