| From: | Alvaro Herrera <alvherre(at)2ndquadrant(dot)com> |
|---|---|
| To: | Peter Geoghegan <pg(at)bowt(dot)ie> |
| Cc: | Jeff Davis <pgsql(at)j-davis(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: logtape.c stats don't account for unused "prefetched" block numbers |
| Date: | 2020-09-01 23:36:02 |
| Message-ID: | 20200901233602.GA31048@alvherre.pgsql |
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| Thread: | |
| Lists: | pgsql-hackers |
On 2020-Jul-30, Peter Geoghegan wrote:
> Commit 896ddf9b added prefetching to logtape.c to avoid excessive
> fragmentation in the context of hash aggs that spill and have many
> batches/tapes. Apparently the preallocation doesn't actually perform
> any filesystem operations, so the new mechanism should be zero
> overhead when "preallocated" blocks aren't actually used after all
> (right?). However, I notice that this breaks the statistics shown by
> things like trace_sort, and even EXPLAIN ANALYZE.
> LogicalTapeSetBlocks() didn't get the memo about preallocation.
This open item hasn't received any replies. I think Peter knows how to
fix it already, but no patch has been posted ... It'd be good to get a
move on it.
--
Álvaro Herrera https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
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