Remember last written LSN when it is first requested #411
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MMeent
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Apr 10, 2024
src/backend/access/transam/xlog.c
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| * SetLastWrittenLSNForRelation -- Set maximal LSN for relation metadata | ||
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Here, too: Not a fan of the additional newline, but won't block it on this.
tristan957
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May 10, 2024
* Remember last written LSN when it is first requested * Use rnode instead of rlocator * Return updated LSN in SetLastWrittenLSN * Remove wrong new line --------- Co-authored-by: Konstantin Knizhnik <[email protected]>
tristan957
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that referenced
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May 20, 2024
* Remember last written LSN when it is first requested * Use rnode instead of rlocator * Return updated LSN in SetLastWrittenLSN * Remove wrong new line --------- Co-authored-by: Konstantin Knizhnik <[email protected]>
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See https://neondb.slack.com/archives/C03QLRH7PPD/p1712529369520409
In case of statements CREATE TABLE AS SELECT... or INSERT FROM SELECT... we are fetching data from source table and storing it in destination table. It cause problems with prefetch last-written-lsn is known for the pages of source table
(which for example happens after compute restart). In this case we get get global value of last-written-lsn which is changed frequently as far as we are writing pages of destination table. As a result request-isn for the prefetch and request-let when this page is actually needed are different and we got exported prefetch request. So it actually disarms prefetch.
Proposed simple patch stores last-written LSN for the page when it is not found. So next time we will request last-written LSN for this page, we will get the same value (certainly if the page was not changed).