Postgres 13

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CREATE TABLE IF NOT EXISTS acls1k (
pkey serial PRIMARY KEY,
user_name text);

INSERT INTO acls1k (pkey,user_name)
SELECT generate_series(1,1000000),substr(md5(random()::text), 0, 25);

CREATE INDEX idx ON acls1k (user_name);

CREATE TABLE

INSERT 0 1000000

CREATE INDEX

EXPLAIN (ANALYSE,COSTS OFF) 
SELECT * FROM acls1k WHERE user_name = 'myuser'::name::text; 

EXPLAIN (ANALYSE,COSTS OFF) 
SELECT * FROM acls1k WHERE user_name = CURRENT_USER::text; 

EXPLAIN (ANALYSE,COSTS OFF) 
WITH u (uname) AS (SELECT CURRENT_USER::text)
SELECT * FROM acls1k a 
JOIN u ON a.user_name = u.uname; 

QUERY PLAN

Gather (actual time=213.359..213.434 rows=0 loops=1)

  Workers Planned: 2

  Workers Launched: 2

  ->  Parallel Seq Scan on acls1k (actual time=193.560..193.560 rows=0 loops=3)

        Filter: (user_name = 'myuser'::text COLLATE "C")

        Rows Removed by Filter: 333333

Planning Time: 0.379 ms

Execution Time: 213.460 ms

EXPLAIN

QUERY PLAN

Gather (actual time=277.119..277.583 rows=0 loops=1)

  Workers Planned: 2

  Workers Launched: 2

  ->  Parallel Seq Scan on acls1k (actual time=254.549..254.549 rows=0 loops=3)

        Filter: (user_name = (CURRENT_USER)::text)

        Rows Removed by Filter: 333333

Planning Time: 0.088 ms

Execution Time: 277.599 ms

EXPLAIN

QUERY PLAN

Gather (actual time=280.458..280.529 rows=0 loops=1)

  Workers Planned: 2

  Workers Launched: 2

  ->  Parallel Seq Scan on acls1k a (actual time=256.719..256.719 rows=0 loops=3)

        Filter: (user_name = (CURRENT_USER)::text)

        Rows Removed by Filter: 333333

Planning Time: 0.104 ms

Execution Time: 280.550 ms

EXPLAIN

PREPARE pu (text) AS SELECT * FROM acls1k WHERE user_name = $1;

EXPLAIN (ANALYSE,COSTS OFF)  EXECUTE pu(CURRENT_USER);

PREPARE

QUERY PLAN

Bitmap Heap Scan on acls1k (actual time=0.058..0.059 rows=0 loops=1)

  Recheck Cond: (user_name = 'u_343360508'::text)

  ->  Bitmap Index Scan on idx (actual time=0.056..0.056 rows=0 loops=1)

        Index Cond: (user_name = 'u_343360508'::text)

Planning Time: 0.201 ms

Execution Time: 0.089 ms

EXPLAIN