I pushed this patch after rewriting it rather completely. I added
tracing notices to inspect the blocks it was prefetching and observed
that the original coding was failing to prefetch the final streak of
blocks in the table, which is an important oversight considering that it
may very well be that those are the only blocks to read at all.
I timed vacuuming a 4000-block table in my laptop (single SSD disk;
dropped FS caches after deleting all rows in table, so that vacuum has
to read all blocks from disk); it changes from 387ms without patch to
155ms with patch. I didn't measure how much it takes to run the other
steps in the vacuum, but it's clear that the speedup for the truncation
phase is considerable.
¡Thanks, Claudio!
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Álvaro Herrera https://www.2ndQuadrant.com/
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