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/search_gin.rs

@@ -0,0 +1,159 @@
+/*
+ * SPDX-FileCopyrightText: 2026 Coffey Labs
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+//! PostgreSQL full-text GIN indexes are built with fastupdate off, and an
+//! index made earlier with the default is switched over at startup. With
+//! fastupdate on, new entries wait in a pending list that every search scans
+//! in full until VACUUM merges it.
+
+use crate::utils::storage::build_data_store;
+use registry::schema::structs::DataStore;
+use store::{Rows, SearchStore, Store};
+
+const SCHEMA: &str = "gin_fastupdate_test";
+
+#[tokio::test(flavor = "multi_thread")]
+pub async fn postgres_gin_fastupdate() {
+    println!("Running PostgreSQL GIN fastupdate test...");
+
+    // Work in a schema of our own so the shared search tables are untouched
+    let admin = Store::build(build_data_store("PostgreSql", "").await)
+        .await
+        .expect("Failed to connect to PostgreSQL");
+    for query in [
+        format!("DROP SCHEMA IF EXISTS {SCHEMA} CASCADE"),
+        format!("CREATE SCHEMA {SCHEMA}"),
+    ] {
+        admin.sql_query::<usize>(&query, vec![]).await.unwrap();
+    }
+    let DataStore::PostgreSql(mut config) = build_data_store("PostgreSql", "").await else {
+        unreachable!()
+    };
+    config.options = Some(format!("-c search_path={SCHEMA}"));
+    let store = Store::build(DataStore::PostgreSql(config))
+        .await
+        .expect("Failed to connect to PostgreSQL");
+    let search = SearchStore::Store(store.clone());
+
+    // A fresh schema
+    search.create_indexes().await.unwrap();
+    let indexes = gin_indexes(&admin).await;
+    assert!(
+        indexes.len() >= 4,
+        "expected the search GIN indexes, found {indexes:?}"
+    );
+    for (name, options) in &indexes {
+        assert!(
+            options.contains("fastupdate=off"),
+            "fresh index {name} has options {options:?}"
+        );
+    }
+
+    // A schema from before the change: the same indexes, made with the
+    // default fastupdate=on, and a pending list with something in it
+    for (name, _) in &indexes {
+        admin
+            .sql_query::<usize>(
+                &format!("ALTER INDEX {SCHEMA}.{name} RESET (fastupdate)"),
+                vec![],
+            )
+            .await
+            .unwrap();
+    }
+    for (name, options) in gin_indexes(&admin).await {
+        assert!(
+            !options.contains("fastupdate"),
+            "index {name} still has options {options:?}"
+        );
+    }
+    admin
+        .sql_query::<usize>(
+            &format!(
+                "INSERT INTO {SCHEMA}.s_email (accid, docid, subj, body) \
+                 SELECT 1, n, to_tsvector('simple', 'pending subject ' || n), \
+                 to_tsvector('simple', 'pending body text ' || n) \
+                 FROM generate_series(1, 500) n"
+            ),
+            vec![],
+        )
+        .await
+        .unwrap();
+    assert!(
+        pending_tuples(&admin, "gin_s_email_body").await > 0,
+        "no pending list to merge"
+    );
+
+    // Startup on the existing schema switches every index over and merges
+    // what was pending
+    search.create_indexes().await.unwrap();
+    for (name, options) in gin_indexes(&admin).await {
+        assert!(
+            options.contains("fastupdate=off"),
+            "existing index {name} has options {options:?} after startup"
+        );
+    }
+    assert_eq!(pending_tuples(&admin, "gin_s_email_body").await, 0);
+
+    // And a second startup changes nothing
+    search.create_indexes().await.unwrap();
+    for (name, options) in gin_indexes(&admin).await {
+        assert!(options.contains("fastupdate=off"), "{name}: {options:?}");
+    }
+
+    admin
+        .sql_query::<usize>(&format!("DROP SCHEMA {SCHEMA} CASCADE"), vec![])
+        .await
+        .unwrap();
+}
+
+/// The GIN indexes in the test schema with their reloptions.
+async fn gin_indexes(admin: &Store) -> Vec<(String, String)> {
+    admin
+        .sql_query::<Rows>(
+            &format!(
+                "SELECT c.relname::text, COALESCE(array_to_string(c.reloptions, ','), '') \
+                 FROM pg_class c JOIN pg_namespace n ON n.oid = c.relnamespace \
+                 JOIN pg_am a ON a.oid = c.relam \
+                 WHERE n.nspname = '{SCHEMA}' AND c.relkind = 'i' AND a.amname = 'gin' \
+                 ORDER BY 1"
+            ),
+            vec![],
+        )
+        .await
+        .unwrap()
+        .rows
+        .into_iter()
+        .map(|row| {
+            let mut values = row.values.into_iter();
+            (
+                values.next().unwrap().to_str().into_owned(),
+                values.next().unwrap().to_str().into_owned(),
+            )
+        })
+        .collect()
+}
+
+/// Tuples waiting in a GIN index's pending list (pgstattuple is a contrib
+/// extension the test database has).
+async fn pending_tuples(admin: &Store, index: &str) -> i64 {
+    admin
+        .sql_query::<usize>("CREATE EXTENSION IF NOT EXISTS pgstattuple", vec![])
+        .await
+        .unwrap();
+    admin
+        .sql_query::<Rows>(
+            &format!("SELECT pending_tuples FROM pgstatginindex('{SCHEMA}.{index}'::regclass)"),
+            vec![],
+        )
+        .await
+        .unwrap()
+        .rows
+        .into_iter()
+        .next()
+        .and_then(|row| row.values.into_iter().next())
+        .map(|value| value.to_str().parse::<i64>().unwrap())
+        .unwrap()
+}