Horton

Re: POC: GROUP BY optimization

jian he <jian.universality@gmail.com>

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From: jian he <jian.universality@gmail.com>
To: Alexander Korotkov <aekorotkov@gmail.com>
Cc: Andrei Lepikhov <a.lepikhov@postgrespro.ru>, Tom Lane <tgl@sss.pgh.pa.us>, Richard Guo <guofenglinux@gmail.com>, Pavel Borisov <pashkin.elfe@gmail.com>, vignesh C <vignesh21@gmail.com>, PostgreSQL Developers <pgsql-hackers@lists.postgresql.org>, Tomas Vondra <tomas.vondra@enterprisedb.com>, Teodor Sigaev <teodor@sigaev.ru>, David Rowley <dgrowleyml@gmail.com>, "a.rybakina" <a.rybakina@postgrespro.ru>
Date: 2024-04-24T06:25:22Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Restore preprocess_groupclause()

  2. Rename PathKeyInfo to GroupByOrdering

  3. Add invariants check to get_useful_group_keys_orderings()

  4. Fix asymmetry in setting EquivalenceClass.ec_sortref

  5. Multiple revisions to the GROUP BY reordering tests

  6. Get rid of pg_class usage in SJE regression tests

  7. Rename index "abc" in aggregates.sql

  8. Explore alternative orderings of group-by pathkeys during optimization.

  9. Generalize the common code of adding sort before processing of grouping

  10. Fix out-dated comment in preprocess_groupclause()

  11. Force parallelism in partition_aggregate

  12. Optimize order of GROUP BY keys

hi.
I found an interesting case.

CREATE TABLE t1 AS
  SELECT (i % 10)::numeric AS x,(i % 10)::int8 AS y,'abc' || i % 10 AS
z, i::int4 AS w
  FROM generate_series(1, 100) AS i;
CREATE INDEX t1_x_y_idx ON t1 (x, y);
ANALYZE t1;
SET enable_hashagg = off;
SET enable_seqscan = off;

EXPLAIN (COSTS OFF) SELECT count(*) FROM t1 GROUP BY x,z,y,w;
EXPLAIN (COSTS OFF) SELECT count(*) FROM t1 GROUP BY x,w,y,z;
EXPLAIN (COSTS OFF) SELECT count(*) FROM t1 GROUP BY x,z,w,y;
EXPLAIN (COSTS OFF) SELECT count(*) FROM t1 GROUP BY x,w,z,y;
the above part will use:
  ->  Incremental Sort
         Sort Key: x, $, $, $
         Presorted Key: x
         ->  Index Scan using t1_x_y_idx on t1

EXPLAIN (COSTS OFF) SELECT count(*) FROM t1 GROUP BY z,y,w,x;
EXPLAIN (COSTS OFF) SELECT count(*) FROM t1 GROUP BY w,y,z,x;
EXPLAIN (COSTS OFF) SELECT count(*) FROM t1 GROUP BY y,z,x,w;
EXPLAIN (COSTS OFF) SELECT count(*) FROM t1 GROUP BY y,w,x,z;
EXPLAIN (COSTS OFF) SELECT count(*) FROM t1 GROUP BY y,x,z,w;
EXPLAIN (COSTS OFF) SELECT count(*) FROM t1 GROUP BY y,x,w,z;

these will use:
  ->  Incremental Sort
         Sort Key: x, y, $, $
         Presorted Key: x, y
         ->  Index Scan using t1_x_y_idx on t1

I guess this is fine, but not optimal?