Обсуждение: WAL reducing size

Is Postgres removes the committed transaction after archive.  LIke in SQL
Server , which removes the committed transactions after taking Log backup.



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how about uncommitted(open transactions) .  if a segment has 1 committed and
2 uncommitted transactions then will it keep the segment instead deleting it
after checkpoint , since it has 2 uncommitted transactions 



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Sent from: http://www.postgresql-archive.org/PostgreSQL-general-f1843780.html

sql2pg wrote:
> how about uncommitted(open transactions) .  if a segment has 1 committed and
> 2 uncommitted transactions then will it keep the segment instead deleting it
> after checkpoint , since it has 2 uncommitted transactions 

There is no connection between transaction commit and checkpoint
removing WAL files.  WAL files are removed after two checkpoints have
occured, regardless of transactions that might be open.  The on-disk
effects of those transactions that occured before the earlier checkpoint
are already on disk anyway.  The commit record might appear at whatever
point in the future, and the rows will not become visible to other
transactions until then, regardless of what happened to the WAL
segments.

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Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

On Tue, Dec 5, 2017 at 6:12 AM, Alvaro Herrera <alvherre@alvh.no-ip.org> wrote:
> sql2pg wrote:
>> how about uncommitted(open transactions) .  if a segment has 1 committed and
>> 2 uncommitted transactions then will it keep the segment instead deleting it
>> after checkpoint , since it has 2 uncommitted transactions
>
> There is no connection between transaction commit and checkpoint
> removing WAL files.  WAL files are removed after two checkpoints have
> occured, regardless of transactions that might be open.  The on-disk
> effects of those transactions that occured before the earlier checkpoint
> are already on disk anyway.  The commit record might appear at whatever
> point in the future, and the rows will not become visible to other
> transactions until then, regardless of what happened to the WAL
> segments.

Something that perhaps does not concern you directly, but in Postgres
11 WAL segments get recycled after one completed checkpoint thanks to
commit 4b0d28d.
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Michael