Re: logtape.c stats don't account for unused "prefetched" block numbers - Mailing list pgsql-hackers

From Alvaro Herrera
Subject Re: logtape.c stats don't account for unused "prefetched" block numbers
Date
Msg-id 20200901233602.GA31048@alvherre.pgsql
Whole thread Raw
In response to logtape.c stats don't account for unused "prefetched" block numbers  (Peter Geoghegan <pg@bowt.ie>)
Responses Re: logtape.c stats don't account for unused "prefetched" block numbers
List 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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