On 01/29/2018 08:21 PM, Vitaliy Garnashevich wrote: > I've read this article: https://wiki.postgresql.org/wiki/Hint_Bits > > It says: > >> A plain SELECT, count(*), or VACUUM on the entire table will check >> every tuple for visibility and set its hint bits. > > Suppose, a new page was created using many INSERTs, and then was written > to disk during a checkpoint. There were no SELECTs or VACUUM on the page > or table yet. Will the following SELECT of one tuple from the page > update hint bits for ALL tuples on the page? Is that correct? > Possibly, if there are no old transactions running. > When a page is initially created and then is being written to disk > during a checkpoint, does checkpoint writer update the hint bits before > writing the page, or the following SELECT/VACUUM will have to do that > (possibly loading/updating/writing the page again)? > Checkpoint only deals with 8kB chunks of data. Hint bits are not set on a page, but on individual items (rows), so it's not up to the checkpoint process to tweak that - that's up to queries accessing the data. regards -- Tomas Vondra http://www.2ndQuadrant.com PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
