Re: Minimal logical decoding on standbys

Hi,

On 2023-01-26 18:56:10 +0100, Drouvot, Bertrand wrote:
> - I'm struggling to create a test for btree killtuples as there is a need for rows removal on the table (that could produce a conflict too):
> Do you've a scenario in mind for this one? (and btw in what kind of WAL record should the conflict be detected in such a case? xl_btree_delete?)

Hm, it might indeed be hard in "modern" postgres. I think you'd need at least
two concurrent sessions, to prevent on-access pruning on the table.

DROP TABLE IF EXISTS indexdel;
CREATE TABLE indexdel(id int8 primary key);
INSERT INTO indexdel SELECT generate_series(1, 10000);
VACUUM indexdel; -- ensure hint bits are set etc

DELETE FROM indexdel;

SELECT pg_current_wal_insert_lsn();

SET enable_indexonlyscan = false;
-- This scan finds that the index items are dead - but doesn't yet issue a
-- btree delete WAL record, that only happens when needing space on the page
-- again.
EXPLAIN (COSTS OFF, SUMMARY OFF) SELECT id FROM indexdel WHERE id < 10 ORDER BY id ASC;
SELECT id FROM indexdel WHERE id < 100 ORDER BY id ASC;

-- The insertions into the range of values prev
INSERT INTO indexdel SELECT generate_series(1, 100);

Does generate the btree deletion record, but it also does emit a PRUNE (from
heapam_index_fetch_tuple() -> heap_page_prune_opt()).

While the session could create a cursor to prevent later HOT cleanup, the
query would also trigger hot pruning (or prevent the rows from being dead, if
you declare the cursor before the DELETE). So you'd need overlapping cursors
in a concurrent session...

Too complicated.

Greetings,

Andres Freund