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c1b7a6c273
We have long forbidden fetching backwards from a NO SCROLL cursor, but the prohibition didn't extend to cases in which we rewind the query altogether and then re-fetch forwards. I think the reason is that this logic was mainly meant to protect plan nodes that can't be run in the reverse direction. However, re-reading the query output is problematic if the query is volatile (which includes SELECT FOR UPDATE, not just queries with volatile functions): the re-read can produce different results, which confuses the cursor navigation logic completely. Another reason for disliking this approach is that some code paths will either fetch backwards or rewind-and-fetch-forwards depending on the distance to the target row; so that seemingly identical use-cases may or may not draw the "cursor can only scan forward" error. Hence, let's clean things up by disallowing rewind as well as fetch-backwards in a NO SCROLL cursor. Ordinarily we'd only make such a definitional change in HEAD, but there is a third reason to consider this change now. Commitba2c6d6cecreated some new user-visible anomalies for non-scrollable cursors WITH HOLD, in that navigation in the cursor result got confused if the cursor had been partially read before committing. The only good way to resolve those anomalies is to forbid rewinding such a cursor, which allows removal of the incorrect cursor state manipulations thatba2c6d6ceadded to PersistHoldablePortal. To minimize the behavioral change in the back branches (including v14), refuse to rewind a NO SCROLL cursor only when it has a holdStore, ie has been held over from a previous transaction due to WITH HOLD. This should avoid breaking most applications that have been sloppy about whether to declare cursors as scrollable. We'll enforce the prohibition across-the-board beginning in v15. Back-patch to v11, asba2c6d6cewas. Discussion: https://postgr.es/m/3712911.1631207435@sss.pgh.pa.us
110 lines
4.2 KiB
PL/PgSQL
110 lines
4.2 KiB
PL/PgSQL
CREATE TABLE test_tablesample (id int, name text) WITH (fillfactor=10);
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-- use fillfactor so we don't have to load too much data to get multiple pages
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INSERT INTO test_tablesample
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SELECT i, repeat(i::text, 200) FROM generate_series(0, 9) s(i);
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SELECT t.id FROM test_tablesample AS t TABLESAMPLE SYSTEM (50) REPEATABLE (0);
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SELECT id FROM test_tablesample TABLESAMPLE SYSTEM (100.0/11) REPEATABLE (0);
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SELECT id FROM test_tablesample TABLESAMPLE SYSTEM (50) REPEATABLE (0);
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SELECT id FROM test_tablesample TABLESAMPLE BERNOULLI (50) REPEATABLE (0);
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SELECT id FROM test_tablesample TABLESAMPLE BERNOULLI (5.5) REPEATABLE (0);
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-- 100% should give repeatable count results (ie, all rows) in any case
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SELECT count(*) FROM test_tablesample TABLESAMPLE SYSTEM (100);
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SELECT count(*) FROM test_tablesample TABLESAMPLE SYSTEM (100) REPEATABLE (1+2);
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SELECT count(*) FROM test_tablesample TABLESAMPLE SYSTEM (100) REPEATABLE (0.4);
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CREATE VIEW test_tablesample_v1 AS
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SELECT id FROM test_tablesample TABLESAMPLE SYSTEM (10*2) REPEATABLE (2);
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CREATE VIEW test_tablesample_v2 AS
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SELECT id FROM test_tablesample TABLESAMPLE SYSTEM (99);
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\d+ test_tablesample_v1
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\d+ test_tablesample_v2
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-- check a sampled query doesn't affect cursor in progress
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BEGIN;
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DECLARE tablesample_cur SCROLL CURSOR FOR
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SELECT id FROM test_tablesample TABLESAMPLE SYSTEM (50) REPEATABLE (0);
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FETCH FIRST FROM tablesample_cur;
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FETCH NEXT FROM tablesample_cur;
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FETCH NEXT FROM tablesample_cur;
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SELECT id FROM test_tablesample TABLESAMPLE SYSTEM (50) REPEATABLE (0);
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FETCH NEXT FROM tablesample_cur;
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FETCH NEXT FROM tablesample_cur;
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FETCH NEXT FROM tablesample_cur;
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FETCH FIRST FROM tablesample_cur;
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FETCH NEXT FROM tablesample_cur;
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FETCH NEXT FROM tablesample_cur;
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FETCH NEXT FROM tablesample_cur;
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FETCH NEXT FROM tablesample_cur;
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FETCH NEXT FROM tablesample_cur;
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CLOSE tablesample_cur;
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END;
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EXPLAIN (COSTS OFF)
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SELECT id FROM test_tablesample TABLESAMPLE SYSTEM (50) REPEATABLE (2);
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EXPLAIN (COSTS OFF)
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SELECT * FROM test_tablesample_v1;
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-- check inheritance behavior
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explain (costs off)
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select count(*) from person tablesample bernoulli (100);
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select count(*) from person tablesample bernoulli (100);
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select count(*) from person;
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-- check that collations get assigned within the tablesample arguments
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SELECT count(*) FROM test_tablesample TABLESAMPLE bernoulli (('1'::text < '0'::text)::int);
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-- check behavior during rescans, as well as correct handling of min/max pct
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select * from
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(values (0),(100)) v(pct),
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lateral (select count(*) from tenk1 tablesample bernoulli (pct)) ss;
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select * from
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(values (0),(100)) v(pct),
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lateral (select count(*) from tenk1 tablesample system (pct)) ss;
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explain (costs off)
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select pct, count(unique1) from
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(values (0),(100)) v(pct),
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lateral (select * from tenk1 tablesample bernoulli (pct)) ss
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group by pct;
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select pct, count(unique1) from
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(values (0),(100)) v(pct),
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lateral (select * from tenk1 tablesample bernoulli (pct)) ss
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group by pct;
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select pct, count(unique1) from
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(values (0),(100)) v(pct),
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lateral (select * from tenk1 tablesample system (pct)) ss
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group by pct;
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-- errors
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SELECT id FROM test_tablesample TABLESAMPLE FOOBAR (1);
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SELECT id FROM test_tablesample TABLESAMPLE SYSTEM (NULL);
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SELECT id FROM test_tablesample TABLESAMPLE SYSTEM (50) REPEATABLE (NULL);
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SELECT id FROM test_tablesample TABLESAMPLE BERNOULLI (-1);
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SELECT id FROM test_tablesample TABLESAMPLE BERNOULLI (200);
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SELECT id FROM test_tablesample TABLESAMPLE SYSTEM (-1);
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SELECT id FROM test_tablesample TABLESAMPLE SYSTEM (200);
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SELECT id FROM test_tablesample_v1 TABLESAMPLE BERNOULLI (1);
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INSERT INTO test_tablesample_v1 VALUES(1);
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WITH query_select AS (SELECT * FROM test_tablesample)
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SELECT * FROM query_select TABLESAMPLE BERNOULLI (5.5) REPEATABLE (1);
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SELECT q.* FROM (SELECT * FROM test_tablesample) as q TABLESAMPLE BERNOULLI (5);
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-- check partitioned tables support tablesample
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create table parted_sample (a int) partition by list (a);
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create table parted_sample_1 partition of parted_sample for values in (1);
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create table parted_sample_2 partition of parted_sample for values in (2);
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explain (costs off)
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select * from parted_sample tablesample bernoulli (100);
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drop table parted_sample, parted_sample_1, parted_sample_2;
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