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CREATE EXTENSION tsm_system_rows;
CREATE TABLE test_tablesample (id int, name text);
INSERT INTO test_tablesample SELECT i, repeat(i::text, 1000)
FROM generate_series(0, 30) s(i);
ANALYZE test_tablesample;
SELECT count(*) FROM test_tablesample TABLESAMPLE system_rows (0);
count
-------
0
(1 row)
SELECT count(*) FROM test_tablesample TABLESAMPLE system_rows (1);
count
-------
1
(1 row)
SELECT count(*) FROM test_tablesample TABLESAMPLE system_rows (10);
count
-------
10
(1 row)
SELECT count(*) FROM test_tablesample TABLESAMPLE system_rows (100);
count
-------
31
(1 row)
-- bad parameters should get through planning, but not execution:
EXPLAIN (COSTS OFF)
SELECT id FROM test_tablesample TABLESAMPLE system_rows (-1);
QUERY PLAN
----------------------------------------
Sample Scan on test_tablesample
Sampling: system_rows ('-1'::bigint)
(2 rows)
SELECT id FROM test_tablesample TABLESAMPLE system_rows (-1);
ERROR: sample size must not be negative
-- fail, this method is not repeatable:
SELECT * FROM test_tablesample TABLESAMPLE system_rows (10) REPEATABLE (0);
ERROR: tablesample method system_rows does not support REPEATABLE
LINE 1: SELECT * FROM test_tablesample TABLESAMPLE system_rows (10) ...
^
-- but a join should be allowed:
EXPLAIN (COSTS OFF)
SELECT * FROM
(VALUES (0),(10),(100)) v(nrows),
LATERAL (SELECT count(*) FROM test_tablesample
TABLESAMPLE system_rows (nrows)) ss;
QUERY PLAN
----------------------------------------------------------
Nested Loop
-> Values Scan on "*VALUES*"
-> Aggregate
-> Sample Scan on test_tablesample
Sampling: system_rows ("*VALUES*".column1)
(5 rows)
SELECT * FROM
(VALUES (0),(10),(100)) v(nrows),
LATERAL (SELECT count(*) FROM test_tablesample
TABLESAMPLE system_rows (nrows)) ss;
nrows | count
-------+-------
0 | 0
10 | 10
100 | 31
(3 rows)
CREATE VIEW vv AS
SELECT count(*) FROM test_tablesample TABLESAMPLE system_rows (20);
SELECT * FROM vv;
count
-------
20
(1 row)
DROP EXTENSION tsm_system_rows; -- fail, view depends on extension
ERROR: cannot drop extension tsm_system_rows because other objects depend on it
DETAIL: view vv depends on function system_rows(internal)
HINT: Use DROP ... CASCADE to drop the dependent objects too.
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