PostgreSQL search GIN indexes without a pending list

A three-node rehearsal on PostgreSQL saw searches take about 185 ms
with 80 to 260 pages in the full-text indexes' pending lists, 2 to 6 ms
right after gin_clean_pending_list() or VACUUM, then creep back up as
mail came in. The search tables' GIN indexes were created with the
default fastupdate=on: new entries wait in an unindexed pending list
that every search scans in full until VACUUM (or 4 MB of backlog)
merges it, and autovacuum only visits an insert-only table after
thousands of inserts.

The search GIN indexes are now created WITH (fastupdate = off), so an
insert pays its index update at once. The schema step runs at every
startup (create_search_tables, via SearchStore::create_indexes), so
indexes made before this change are switched there: when an index's
reloptions don't already turn fastupdate off, ALTER INDEX ... SET
(fastupdate = off) and one gin_clean_pending_list() merge its backlog.
The ALTER takes a SHARE UPDATE EXCLUSIVE lock, which blocks neither
reads nor writes; after the first startup the step is one catalog read
per index. A failure is logged and startup goes on (search still
works, only slower).

Per-table autovacuum settings for the search tables are left alone.
The pending list was the only reason the insert threshold mattered for
search; dead tuples and freezing are served by the defaults, and table
settings would override whatever tuning the DBA has done.

MySQL is unaffected: InnoDB FULLTEXT keeps new entries in an in-memory
cache that queries read directly, with no setting like fastupdate.

store::search_gin::postgres_gin_fastupdate (new, PostgreSQL) builds
the search schema in a schema of its own and checks pg_class.reloptions:
fastupdate=off on every GIN index of a fresh schema; then, with the
option reset to the default and 500 rows pending, one startup turns it
off everywhere and leaves no pending tuples (pgstatginindex); a second
startup changes nothing. On main it fails at the first check.

tests/src/store/mod.rs

@@ -21,6 +21,8 @@ pub mod replica_mysql; // inbuxa: read replicas on MySQL
 #[cfg(all(feature = "postgres", feature = "redis"))]
 pub mod replica_cluster; // inbuxa: read replicas across nodes
 pub mod scaleout; // inbuxa: scale-out storage
+#[cfg(feature = "postgres")]
+pub mod search_gin; // inbuxa: GIN indexes without a pending list
 #[cfg(any(feature = "postgres", feature = "mysql"))]
 pub mod sql_timeout;
 pub mod task_locks; // inbuxa: task locks across nodes