Skip to content

Commit dd7a8f6

Browse files
tglsfdctglsfdc
committed
Redesign tablesample method API, and do extensive code review.
The original implementation of TABLESAMPLE modeled the tablesample method API on index access methods, which wasn't a good choice because, without specialized DDL commands, there's no way to build an extension that can implement a TSM. (Raw inserts into system catalogs are not an acceptable thing to do, because we can't undo them during DROP EXTENSION, nor will pg_upgrade behave sanely.) Instead adopt an API more like procedural language handlers or foreign data wrappers, wherein the only SQL-level support object needed is a single handler function identified by having a special return type. This lets us get rid of the supporting catalog altogether, so that no custom DDL support is needed for the feature. Adjust the API so that it can support non-constant tablesample arguments (the original coding assumed we could evaluate the argument expressions at ExecInitSampleScan time, which is undesirable even if it weren't outright unsafe), and discourage sampling methods from looking at invisible tuples. Make sure that the BERNOULLI and SYSTEM methods are genuinely repeatable within and across queries, as required by the SQL standard, and deal more honestly with methods that can't support that requirement. Make a full code-review pass over the tablesample additions, and fix assorted bugs, omissions, infelicities, and cosmetic issues (such as failure to put the added code stanzas in a consistent ordering). Improve EXPLAIN's output of tablesample plans, too. Back-patch to 9.5 so that we don't have to support the original API in production.
1 parent b26e3d6 commit dd7a8f6

Some content is hidden

Large Commits have some content hidden by default. Use the searchbox below for content that may be hidden.

83 files changed

+3116
-2521
lines changed

‎contrib/pg_stat_statements/pg_stat_statements.c

Lines changed: 10 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -2297,6 +2297,7 @@ JumbleRangeTable(pgssJumbleState *jstate, List *rtable)
22972297
{
22982298
case RTE_RELATION:
22992299
APP_JUMB(rte->relid);
2300+
JumbleExpr(jstate, (Node *) rte->tablesample);
23002301
break;
23012302
case RTE_SUBQUERY:
23022303
JumbleQuery(jstate, rte->subquery);
@@ -2767,6 +2768,15 @@ JumbleExpr(pgssJumbleState *jstate, Node *node)
27672768
JumbleExpr(jstate, rtfunc->funcexpr);
27682769
}
27692770
break;
2771+
case T_TableSampleClause:
2772+
{
2773+
TableSampleClause *tsc = (TableSampleClause *) node;
2774+
2775+
APP_JUMB(tsc->tsmhandler);
2776+
JumbleExpr(jstate, (Node *) tsc->args);
2777+
JumbleExpr(jstate, (Node *) tsc->repeatable);
2778+
}
2779+
break;
27702780
default:
27712781
/* Only a warning, since we can stumble along anyway */
27722782
elog(WARNING, "unrecognized node type: %d",

‎contrib/tsm_system_rows/Makefile

Lines changed: 2 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -1,8 +1,8 @@
1-
# src/test/modules/tsm_system_rows/Makefile
1+
# contrib/tsm_system_rows/Makefile
22

33
MODULE_big = tsm_system_rows
44
OBJS = tsm_system_rows.o $(WIN32RES)
5-
PGFILEDESC = "tsm_system_rows - SYSTEM TABLESAMPLE method which accepts number of rows as a limit"
5+
PGFILEDESC = "tsm_system_rows - TABLESAMPLE method which accepts number of rows as a limit"
66

77
EXTENSION = tsm_system_rows
88
DATA = tsm_system_rows--1.0.sql

‎contrib/tsm_system_rows/expected/tsm_system_rows.out

Lines changed: 74 additions & 22 deletions
Original file line numberDiff line numberDiff line change
@@ -1,31 +1,83 @@
11
CREATE EXTENSION tsm_system_rows;
2-
CREATE TABLE test_tablesample (id int, name text) WITH (fillfactor=10); -- force smaller pages so we don't have to load too much data to get multiple pages
3-
INSERT INTO test_tablesample SELECT i, repeat(i::text, 1000) FROM generate_series(0, 30) s(i) ORDER BY i;
2+
CREATE TABLE test_tablesample (id int, name text);
3+
INSERT INTO test_tablesample SELECT i, repeat(i::text, 1000)
4+
FROM generate_series(0, 30) s(i);
45
ANALYZE test_tablesample;
5-
SELECT count(*) FROM test_tablesample TABLESAMPLE system_rows (1000);
6+
SELECT count(*) FROM test_tablesample TABLESAMPLE system_rows (0);
7+
count
8+
-------
9+
0
10+
(1 row)
11+
12+
SELECT count(*) FROM test_tablesample TABLESAMPLE system_rows (1);
13+
count
14+
-------
15+
1
16+
(1 row)
17+
18+
SELECT count(*) FROM test_tablesample TABLESAMPLE system_rows (10);
19+
count
20+
-------
21+
10
22+
(1 row)
23+
24+
SELECT count(*) FROM test_tablesample TABLESAMPLE system_rows (100);
625
count
726
-------
827
31
928
(1 row)
1029

11-
SELECT id FROM test_tablesample TABLESAMPLE system_rows (8) REPEATABLE (5432);
12-
id
13-
----
14-
7
15-
14
16-
21
17-
28
18-
4
19-
11
20-
18
21-
25
22-
(8 rows)
23-
24-
EXPLAIN SELECT id FROM test_tablesample TABLESAMPLE system_rows (20) REPEATABLE (10);
25-
QUERY PLAN
26-
-----------------------------------------------------------------------------------
27-
Sample Scan (system_rows) on test_tablesample (cost=0.00..80.20 rows=20 width=4)
30+
-- bad parameters should get through planning, but not execution:
31+
EXPLAIN (COSTS OFF)
32+
SELECT id FROM test_tablesample TABLESAMPLE system_rows (-1);
33+
QUERY PLAN
34+
----------------------------------------
35+
Sample Scan on test_tablesample
36+
Sampling: system_rows ('-1'::bigint)
37+
(2 rows)
38+
39+
SELECT id FROM test_tablesample TABLESAMPLE system_rows (-1);
40+
ERROR: sample size must not be negative
41+
-- fail, this method is not repeatable:
42+
SELECT * FROM test_tablesample TABLESAMPLE system_rows (10) REPEATABLE (0);
43+
ERROR: tablesample method system_rows does not support REPEATABLE
44+
LINE 1: SELECT * FROM test_tablesample TABLESAMPLE system_rows (10) ...
45+
^
46+
-- but a join should be allowed:
47+
EXPLAIN (COSTS OFF)
48+
SELECT * FROM
49+
(VALUES (0),(10),(100)) v(nrows),
50+
LATERAL (SELECT count(*) FROM test_tablesample
51+
TABLESAMPLE system_rows (nrows)) ss;
52+
QUERY PLAN
53+
----------------------------------------------------------
54+
Nested Loop
55+
-> Values Scan on "*VALUES*"
56+
-> Aggregate
57+
-> Sample Scan on test_tablesample
58+
Sampling: system_rows ("*VALUES*".column1)
59+
(5 rows)
60+
61+
SELECT * FROM
62+
(VALUES (0),(10),(100)) v(nrows),
63+
LATERAL (SELECT count(*) FROM test_tablesample
64+
TABLESAMPLE system_rows (nrows)) ss;
65+
nrows | count
66+
-------+-------
67+
0 | 0
68+
10 | 10
69+
100 | 31
70+
(3 rows)
71+
72+
CREATE VIEW vv AS
73+
SELECT count(*) FROM test_tablesample TABLESAMPLE system_rows (20);
74+
SELECT * FROM vv;
75+
count
76+
-------
77+
20
2878
(1 row)
2979

30-
-- done
31-
DROP TABLE test_tablesample CASCADE;
80+
DROP EXTENSION tsm_system_rows; -- fail, view depends on extension
81+
ERROR: cannot drop extension tsm_system_rows because other objects depend on it
82+
DETAIL: view vv depends on function system_rows(internal)
83+
HINT: Use DROP ... CASCADE to drop the dependent objects too.

‎contrib/tsm_system_rows/sql/tsm_system_rows.sql

Lines changed: 33 additions & 8 deletions
Original file line numberDiff line numberDiff line change
@@ -1,14 +1,39 @@
11
CREATE EXTENSION tsm_system_rows;
22

3-
CREATE TABLE test_tablesample (id int, name text) WITH (fillfactor=10); -- force smaller pages so we don't have to load too much data to get multiple pages
4-
5-
INSERT INTO test_tablesample SELECT i, repeat(i::text, 1000) FROM generate_series(0, 30) s(i) ORDER BY i;
3+
CREATE TABLE test_tablesample (id int, name text);
4+
INSERT INTO test_tablesample SELECT i, repeat(i::text, 1000)
5+
FROM generate_series(0, 30) s(i);
66
ANALYZE test_tablesample;
77

8-
SELECT count(*) FROM test_tablesample TABLESAMPLE system_rows (1000);
9-
SELECT id FROM test_tablesample TABLESAMPLE system_rows (8) REPEATABLE (5432);
8+
SELECT count(*) FROM test_tablesample TABLESAMPLE system_rows (0);
9+
SELECT count(*) FROM test_tablesample TABLESAMPLE system_rows (1);
10+
SELECT count(*) FROM test_tablesample TABLESAMPLE system_rows (10);
11+
SELECT count(*) FROM test_tablesample TABLESAMPLE system_rows (100);
12+
13+
-- bad parameters should get through planning, but not execution:
14+
EXPLAIN (COSTS OFF)
15+
SELECT id FROM test_tablesample TABLESAMPLE system_rows (-1);
16+
17+
SELECT id FROM test_tablesample TABLESAMPLE system_rows (-1);
18+
19+
-- fail, this method is not repeatable:
20+
SELECT * FROM test_tablesample TABLESAMPLE system_rows (10) REPEATABLE (0);
21+
22+
-- but a join should be allowed:
23+
EXPLAIN (COSTS OFF)
24+
SELECT * FROM
25+
(VALUES (0),(10),(100)) v(nrows),
26+
LATERAL (SELECT count(*) FROM test_tablesample
27+
TABLESAMPLE system_rows (nrows)) ss;
28+
29+
SELECT * FROM
30+
(VALUES (0),(10),(100)) v(nrows),
31+
LATERAL (SELECT count(*) FROM test_tablesample
32+
TABLESAMPLE system_rows (nrows)) ss;
33+
34+
CREATE VIEW vv AS
35+
SELECT count(*) FROM test_tablesample TABLESAMPLE system_rows (20);
1036

11-
EXPLAIN SELECT id FROM test_tablesample TABLESAMPLE system_rows (20) REPEATABLE (10);
37+
SELECT * FROM vv;
1238

13-
-- done
14-
DROP TABLE test_tablesample CASCADE;
39+
DROP EXTENSION tsm_system_rows; -- fail, view depends on extension

‎contrib/tsm_system_rows/tsm_system_rows--1.0.sql

Lines changed: 4 additions & 39 deletions
Original file line numberDiff line numberDiff line change
@@ -1,44 +1,9 @@
1-
/* src/test/modules/tablesample/tsm_system_rows--1.0.sql */
1+
/* contrib/tsm_system_rows/tsm_system_rows--1.0.sql */
22

33
-- complain if script is sourced in psql, rather than via CREATE EXTENSION
44
\echo Use "CREATE EXTENSION tsm_system_rows" to load this file. \quit
55

6-
CREATE FUNCTION tsm_system_rows_init(internal, int4, int4)
7-
RETURNS void
8-
AS 'MODULE_PATHNAME'
6+
CREATE FUNCTION system_rows(internal)
7+
RETURNS tsm_handler
8+
AS 'MODULE_PATHNAME', 'tsm_system_rows_handler'
99
LANGUAGE C STRICT;
10-
11-
CREATE FUNCTION tsm_system_rows_nextblock(internal)
12-
RETURNS int4
13-
AS 'MODULE_PATHNAME'
14-
LANGUAGE C STRICT;
15-
16-
CREATE FUNCTION tsm_system_rows_nexttuple(internal, int4, int2)
17-
RETURNS int2
18-
AS 'MODULE_PATHNAME'
19-
LANGUAGE C STRICT;
20-
21-
CREATE FUNCTION tsm_system_rows_examinetuple(internal, int4, internal, bool)
22-
RETURNS bool
23-
AS 'MODULE_PATHNAME'
24-
LANGUAGE C STRICT;
25-
26-
CREATE FUNCTION tsm_system_rows_end(internal)
27-
RETURNS void
28-
AS 'MODULE_PATHNAME'
29-
LANGUAGE C STRICT;
30-
31-
CREATE FUNCTION tsm_system_rows_reset(internal)
32-
RETURNS void
33-
AS 'MODULE_PATHNAME'
34-
LANGUAGE C STRICT;
35-
36-
CREATE FUNCTION tsm_system_rows_cost(internal, internal, internal, internal, internal, internal, internal)
37-
RETURNS void
38-
AS 'MODULE_PATHNAME'
39-
LANGUAGE C STRICT;
40-
41-
INSERT INTO pg_tablesample_method VALUES('system_rows', false, true,
42-
'tsm_system_rows_init', 'tsm_system_rows_nextblock',
43-
'tsm_system_rows_nexttuple', 'tsm_system_rows_examinetuple',
44-
'tsm_system_rows_end', 'tsm_system_rows_reset', 'tsm_system_rows_cost');

0 commit comments

Comments
 (0)