Hi hackers,
Heap tuple visibility hint bits avoid repeated transaction status
lookups. With wal_log_hints enabled, PostgreSQL currently logs a
full-page image for the first hint bit change to a page after each
checkpoint. This can generate considerably more WAL than the hint bits
themselves require.
My main motivation comes from our PostgreSQL fork with compute/storage
separation, where this WAL volume is costly. The same overhead also
occurs in vanilla clusters with wal_log_hints enabled.
The concept is mostly viable when torn pages are not a problem.
The attached patch records heap tuple offsets and visibility hint bits
in a compact WAL record when checksums are disabled. WAL replay applies
the hints on standbys. The record does not advance the page LSN, so a
later ordinary change still generates the FPI required by
full_page_writes. Checksummed pages continue to use FPI_FOR_HINT.
For the first scan of one million rows, WAL fell from 36.8 MB to 4.2 MB
for an int-only table and from 134.7 MB to 4.9 MB for a
pgbench_accounts-shaped table. With wal_compression=lz4, WAL fell from
13.9 MB to 4.2 MB and from 16.3 MB to 4.9 MB, respectively. About 3.3x in
both cases.
PFA.
Best regards, Andrey Borodin.
