Re: WAL reducing size

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.
-- 
Michael