Subject: [PATCH] Reduce pg_class scans in GRANT/REVOKE ON ALL TABLES IN SCHEMA

From: CharSyam <[email-masked]>
Date: Sun, 12 Apr 2026 16:22:24 +0900

Hi hackers,

  While reading aclchk.c I noticed that objectsInSchemaToOids(), used
  by GRANT/REVOKE ... ON ALL TABLES IN SCHEMA, calls
  getRelationsInNamespace() five times for the OBJECT_TABLE case --
  once per relkind (RELATION, VIEW, MATVIEW, FOREIGN_TABLE,
  PARTITIONED_TABLE):

      case OBJECT_TABLE:
          objs = getRelationsInNamespace(namespaceId, RELKIND_RELATION);
          objects = list_concat(objects, objs);
          objs = getRelationsInNamespace(namespaceId, RELKIND_VIEW);
          ...
          objs = getRelationsInNamespace(namespaceId,
RELKIND_PARTITIONED_TABLE);
          objects = list_concat(objects, objs);
          break;

  pg_class does have an index on (relname, relnamespace), but there
  is no index matching (relnamespace, relkind), so each of those
  per-relkind calls falls back to a full heap scan via
  table_beginscan_catalog().  The work is just repeated five times.

  The attached patch introduces a small helper
  getRelationsInNamespaceMulti() that performs a single heap scan
  filtered by relnamespace and distributes matching tuples into
  per-relkind buckets supplied by the caller.  Relkind filtering is
  done in C after each tuple is read, which is trivially cheap.  The
  OBJECT_TABLE case uses the helper; OBJECT_SEQUENCE and
  OBJECT_PROPGRAPH are left on the original getRelationsInNamespace()
  helper because they only need a single relkind and benefit from the
  second ScanKey.

  Correctness / order preservation
  --------------------------------
   * Result order is identical.  The underlying pg_class heap (and
     thus its physical scan order) is the same regardless of how we
     filter, so each bucket ends up holding the same OIDs in the same
     relative order as a separate per-relkind heap scan would have
     produced.  Concatenating the buckets in the original relkind
     order reproduces the previous list tuple-for-tuple.

     I verified this empirically.  On a schema with interleaved
     relkinds (tables, views, matviews, partitioned tables) I ran two
     equivalent SQL formulations while forcing seq scans on pg_class:

       OLD-path model: UNION ALL of five
         "SELECT oid FROM pg_class
            WHERE relnamespace = X AND relkind = Y
            ORDER BY ctid"
         queries, one per relkind, in the same group order the code
         uses.

       NEW-path model: a single
         "SELECT oid FROM pg_class
            WHERE relnamespace = X
            ORDER BY ctid"
         bucketed by relkind and concatenated in the same group
         order.

     The two formulations produced identical OID sequences, element
     by element.  A positional FULL JOIN between them returned zero
     rows.

   * MVCC semantics are, if anything, a bit stricter.  The old code
     took five separate catalog scans, so in principle concurrent DDL
     could commit between scan N and scan N+1 and be visible to one
     but not another.  With a single scan everything is collected
     under one catalog snapshot.

   * Locking is unchanged in kind: AccessShareLock on pg_class is
     still taken, just once instead of five times.

  Benchmark
  ---------
  This is a targeted micro-optimization, not a dramatic speedup.
  With 10,000 tables in a single schema (pg_class ~10,452 rows),
  running GRANT/REVOKE SELECT ON ALL TABLES IN SCHEMA in a loop
  (6 iterations, first dropped as warmup), I measured a consistent
  ~15% reduction in end-to-end time:

                       baseline    patched     delta
      GRANT  (avg)     88.2 ms     75.9 ms    -14%
      REVOKE (avg)    134.9 ms    115.7 ms    -14%

  Per-iteration numbers (ms):

      baseline GRANT : 92, 87, 87, 85, 89
      patched  GRANT : 77, 72, 72, 76, 79, 79
      baseline REVOKE: 145, 144, 132, 128, 130, 128
      patched  REVOKE: 114, 117, 112, 120, 112, 119

  The absolute savings are small because most of the time in these
  commands is spent updating per-relation ACL tuples, not scanning
  pg_class.  For schemas with only a handful of relations the effect
  is not measurable.  The change is aimed at multi-tenant /
  partition-heavy installations that regularly issue
  "... ON ALL TABLES IN SCHEMA ..." statements over large catalogs.

  Testing
  -------
  Both `make check` and `make check-world` pass cleanly with the
  patch applied on top of current master (all suites green, no new
  failures).  TAP tests were not exercised (tree configured without
  --enable-tap-tests); I can rerun with TAP enabled if that is useful.

  The patch is attached (against master).  Feedback and review
  welcome -- in particular I'd like to know if anyone sees a
  correctness concern I missed, or prefers a different shape for the
  helper (e.g. returning a single flat list rather than per-relkind
  buckets).

  Thanks,
  charsyam

---

From: Michael Paquier <[email-masked]>
Date: Mon, 13 Apr 2026 09:43:00 +0900

>       REVOKE (avg)    134.9 ms    115.7 ms    -14%

I am pretty sure that there are users with millions of relations in a
single schema that could benefit from that.  At least that would not
be surprising with partitioning these days, and foreign tables.  What
kind of numbers do you get if you bump up the number of digits for
these tests.  Let's say a comparison based on a few million relations
at least?

The change you are proposing looks simple enough, quickly skimming
through the patch.  There may be more optimizations doable here, I
have not looked at that, still I tend to like such micro-optimization
proposals as they provide a silent benefit.
--
Michael

---

From: CharSyam <[email-masked]>
Date: Mon, 13 Apr 2026 18:52:57 +0900

I ran a benchmark on the patch *Reduce pg_class scans in GRANT/REVOKE
  ON ALL TABLES IN SCHEMA* (collapses 5 per-relkind pg_class heap scans
  into 1 scan distributed into per-relkind buckets). Summary below.(It took
much time to tests)

  ## Assumptions

  - Two builds of PostgreSQL 19devel from the same source tree (one
    patched, one at master tip), identical compile flags, separate
    --prefix.
  - Separate data directories, run sequentially on an otherwise idle
    host.
  - GUCs: shared_buffers=2GB, max_locks_per_transaction=100000,
    fsync=off, synchronous_commit=off, full_page_writes=off,
    autovacuum=off.
  - bench_s schema contains N empty tables (CREATE TABLE t_i()).
  - Measured operations:
      GRANT  SELECT ON ALL TABLES IN SCHEMA bench_s TO   bench_role
      REVOKE ALL PRIVILEGES ON ALL TABLES IN SCHEMA bench_s FROM bench_role
  - Best of 3 runs reported (seconds).
  - Two scenarios:
      A. Clean catalog   — VACUUM FULL pg_class; VACUUM FULL pg_attribute
                           immediately before measurement. pg_class
                           densely packed.
      B. Bloated catalog — pre-bloat phase: GRANT+REVOKE on all N tables
                           repeated C cycles, no VACUUM. Both patched
                           and master operate on catalogs with identical
                           relpages and n_dead_tup.

  ## Results — Scenario A (Clean catalog)

  macOS (Apple Silicon), best of 3, seconds:

    ntables | GRANT patched | GRANT master | REVOKE patched | REVOKE master
    --------+---------------+--------------+----------------+--------------
     20,000 |     0.116     |    0.115     |     0.231      |    0.226
     40,000 |     0.250     |    0.250     |     0.460      |    0.460
    100,000 |     0.730     |    0.678     |     1.193      |    1.193

  Honestly, there is no measurable performance difference in the clean
  state. Patched and master are statistically indistinguishable within
  run-to-run noise. This matches the design of the patch: when pg_class
  is densely packed, repeating a small seq scan five times is cheap, so
  collapsing it into one has nothing meaningful to save. The patch adds
  no overhead either — worst case is a tie.

  ## Results — Scenario B (Bloated catalog)

  ### Linux x86_64, C=20, best of 3, seconds

    ntables   | dead_tup  | GRANT patched | GRANT master |   Δ     | REVOKE
patched | REVOKE master |   Δ

----------+-----------+---------------+--------------+---------+----------------+---------------+---------
       10,000 |         0 |    0.0924     |    0.0935    |  −1.2 % |
0.1668     |    0.1696     |  −1.6 %
       20,000 |   109,825 |    0.2027     |    0.2069    |  −2.0 % |
0.3381     |    0.3533     |  −4.3 %
       50,000 |   329,468 |    0.5555     |    0.5895    |  −5.8 % |
0.8901     |    0.9371     |  −5.0 %
      100,000 |   879,311 |    1.1732     |    1.1968    |  −2.0 % |
1.8808     |    1.9555     |  −3.8 %
      200,000 | 1,978,925 |    2.2188     |    2.3470    |  −5.5 % |
3.7290     |    3.9064     |  −4.5 %
      500,000 | 4,178,604 |    6.0260     |    6.6663    |  −9.6 % |
9.8162     |   10.2169     |  −3.9 %
    1,000,000 | 9,678,399 |   12.9241     |   14.7657    | −12.5 % |
 24.8893     |   28.7566     | −13.4 %

  ### macOS (Apple Silicon), C=20 (C=10 at 1M), best of 3, seconds

    ntables   | dead_tup  | GRANT patched | GRANT master |   Δ    | REVOKE
patched | REVOKE master |   Δ

----------+-----------+---------------+--------------+--------+----------------+---------------+--------
       20,000 |   299,960 |    0.168      |    0.163     |  +3 %  |
0.260      |    0.278      |  −6 %
       40,000 |   519,601 |    0.307      |    0.307     |   0 %  |
0.552      |    0.564      |  −2 %
      100,000 |   959,268 |    0.784      |    0.934     | −16 %  |
1.405      |    1.419      |  ~0 %
      200,000 | 2,058,886 |    1.787      |    1.878     |  −5 %  |
2.745      |    2.849      |  −4 %
      500,000 | 4,258,565 |    4.727      |    5.197     |  −9 %  |
7.126      |    7.908      | −10 %
    1,000,000 | 9,758,364 |   10.977      |   11.126     |  −1 %  |
 19.473      |   20.759      |  −6 %

  Negative Δ = patched faster. Under catalog bloat the patch produces a
  consistent, reproducible improvement on both operating systems.

  Happy to share the bench scripts and raw logs on request.

  Thanks,
  charsyam