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Hint bits: why the first read after a write dirties pages

Simple terms

You finish a big COPY, the load is committed, and the very first SELECT over those rows is slow, and dirties pages, on a query that writes nothing. Run it again and it's instant. To judge whether a row is visible Postgres has to know if the writing transaction committed, and that means a lookup in the commit log. Doing that per row, per scan, forever would be brutal. So the first reader writes the answer onto the row itself. That first scan is paying for every scan after it.

You might be asked

Right after a big COPY/INSERT load that has already committed, the very FIRST SELECT over those rows is slow and -- surprisingly -- generates a lot of dirty pages and WAL-ish write activity, even though it's a read-only query that changes no data. Run it a second time and it's fast and clean. Explain what's happening at the tuple level.

TopicHint bits: why the first read after a write consults clog and dirties pages (HEAP_XMIN_COMMITTED) even though it changed no data
PostgreSQL17.10 (the hint-bit mechanism and the WAL-flush interlock are unchanged on 12-17)
Tools usedpgi17 lab (PostgreSQL 17.10), pageinspect (heap_page_items.t_infomask, get_raw_page), source @REL_17_10 src/backend/access/heap/heapam_visibility.c + src/include/access/htup_details.h
Last reviewed2026-06-19

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