Index LP_DEAD pruning (btree kill_prior_tuple)
Simple terms
VACUUM is not the only thing keeping an index from bloating. When a scan follows an index entry down to a row that is dead to every running transaction, it quietly flags that entry as it goes, and a later insert on the page reuses the space. Ordinary SELECTs are tidying up behind you. But leave one old transaction open and the row is no longer dead to everyone, the flag never gets set, nothing gets reused, and the index grows while you watch.
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After you DELETE or UPDATE a lot of rows but BEFORE autovacuum runs, what removes the now-dead entries from a B-tree index, and what actually triggers it? Walk me through how a plain SELECT can shrink an index's logical garbage with no VACUUM, why that same read sometimes does nothing at all, and how this keeps a heavily-churned index from bloating.
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