COUNT(*) vs COUNT(1) in PostgreSQL

count-asterisk-vs-count-1-postgres.sql

-- Copyright Data Geekery GmbH
--
-- Licensed under the Apache License, Version 2.0 (the "License");
-- you may not use this file except in compliance with the License.
-- You may obtain a copy of the License at
--
--     http://www.apache.org/licenses/LICENSE-2.0
--
-- Unless required by applicable law or agreed to in writing, software
-- distributed under the License is distributed on an "AS IS" BASIS,
-- WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-- See the License for the specific language governing permissions and
-- limitations under the License.
--
CREATE TABLE t (i INT);
INSERT INTO t SELECT i FROM generate_series(1, 1000000) AS d(i);

DO $$
DECLARE
  v_ts TIMESTAMP;
  v_repeat CONSTANT INT := 100;
  rec RECORD;
  run INT[];
  stmt INT[];
  elapsed DECIMAL[];
  min_elapsed DECIMAL;
  i INT := 1;
BEGIN

  -- Repeat the whole benchmark several times to avoid warmup penalty
  FOR r IN 1..5 LOOP
    v_ts := clock_timestamp();

    FOR i IN 1..v_repeat LOOP
      FOR rec IN (
        SELECT COUNT(*) FROM t
      ) LOOP
        NULL;
      END LOOP;
    END LOOP;

    run[i] := r;
    stmt[i] := 1;
    elapsed[i] := (EXTRACT(EPOCH FROM CAST(clock_timestamp() AS TIMESTAMP)) - EXTRACT(EPOCH FROM v_ts));
    i := i + 1;
    v_ts := clock_timestamp();

    FOR i IN 1..v_repeat LOOP
      FOR rec IN (
        SELECT COUNT(1) FROM t
      ) LOOP
        NULL;
      END LOOP;
    END LOOP;

    run[i] := r;
    stmt[i] := 2;
    elapsed[i] := (EXTRACT(EPOCH FROM CAST(clock_timestamp() AS TIMESTAMP)) - EXTRACT(EPOCH FROM v_ts));
    i := i + 1;
  END LOOP;

  SELECT min(t.elapsed)
  INTO min_elapsed
  FROM unnest(elapsed) AS t(elapsed);

  FOR i IN 1..array_length(run, 1) LOOP
    RAISE INFO 'RUN %, Statement %: %', run[i], stmt[i], CAST(elapsed[i] / min_elapsed AS DECIMAL(10, 5));
  END LOOP;
  
  RAISE INFO '';
  RAISE INFO 'Copyright Data Geekery GmbH';
  RAISE INFO 'https://www.jooq.org/benchmark';
END$$;

DROP TABLE t;