Remember last written LSN when it is first requested #410
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MMeent
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Apr 10, 2024
tristan957
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* Remember last written LSN when it is first requested * Return updated LSN in SetLastWrittenLSN --------- Co-authored-by: Konstantin Knizhnik <knizhnik@neon.tech>
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See https://neondb.slack.com/archives/C03QLRH7PPD/p1712529369520409
In case of statements
CREATE TABLE AS SELECT...orINSERT 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).