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Add regression tests exercising more code paths in nodeLimit.c.
Perusal of the code coverage report shows that the existing regression test cases for LIMIT/OFFSET don't exercise the nodeLimit code paths involving backwards scan, empty results, or null values of LIMIT/OFFSET. Improve the coverage.
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‎src/test/regress/expected/limit.out‎

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@@ -108,6 +108,185 @@ SELECT ''::text AS five, unique1, unique2, stringu1
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| 904 | 793 | UIAAAA
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(5 rows)
110110

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-- Test null limit and offset. The planner would discard a simple null
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-- constant, so to ensure executor is exercised, do this:
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select * from int8_tbl limit (case when random() < 0.5 then null::bigint end);
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q1 | q2
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------------------+-------------------
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123 | 456
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123 | 4567890123456789
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4567890123456789 | 123
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4567890123456789 | 4567890123456789
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4567890123456789 | -4567890123456789
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(5 rows)
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select * from int8_tbl offset (case when random() < 0.5 then null::bigint end);
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q1 | q2
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------------------+-------------------
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123 | 456
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123 | 4567890123456789
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4567890123456789 | 123
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4567890123456789 | 4567890123456789
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4567890123456789 | -4567890123456789
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(5 rows)
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-- Test assorted cases involving backwards fetch from a LIMIT plan node
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begin;
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declare c1 cursor for select * from int8_tbl limit 10;
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fetch all in c1;
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q1 | q2
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------------------+-------------------
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123 | 456
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123 | 4567890123456789
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4567890123456789 | 123
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4567890123456789 | 4567890123456789
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4567890123456789 | -4567890123456789
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(5 rows)
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fetch 1 in c1;
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q1 | q2
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----+----
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(0 rows)
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fetch backward 1 in c1;
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q1 | q2
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------------------+-------------------
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4567890123456789 | -4567890123456789
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(1 row)
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fetch backward all in c1;
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q1 | q2
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------------------+------------------
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4567890123456789 | 4567890123456789
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4567890123456789 | 123
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123 | 4567890123456789
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123 | 456
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(4 rows)
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fetch backward 1 in c1;
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q1 | q2
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----+----
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(0 rows)
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fetch all in c1;
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q1 | q2
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------------------+-------------------
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123 | 456
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123 | 4567890123456789
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4567890123456789 | 123
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4567890123456789 | 4567890123456789
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4567890123456789 | -4567890123456789
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(5 rows)
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declare c2 cursor for select * from int8_tbl limit 3;
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fetch all in c2;
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q1 | q2
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------------------+------------------
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123 | 456
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123 | 4567890123456789
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4567890123456789 | 123
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(3 rows)
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fetch 1 in c2;
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q1 | q2
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----+----
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(0 rows)
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fetch backward 1 in c2;
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q1 | q2
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------------------+-----
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4567890123456789 | 123
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(1 row)
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fetch backward all in c2;
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q1 | q2
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-----+------------------
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123 | 4567890123456789
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123 | 456
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(2 rows)
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fetch backward 1 in c2;
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q1 | q2
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----+----
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(0 rows)
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fetch all in c2;
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q1 | q2
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------------------+------------------
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123 | 456
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123 | 4567890123456789
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4567890123456789 | 123
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(3 rows)
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declare c3 cursor for select * from int8_tbl offset 3;
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fetch all in c3;
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q1 | q2
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------------------+-------------------
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4567890123456789 | 4567890123456789
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4567890123456789 | -4567890123456789
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(2 rows)
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fetch 1 in c3;
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q1 | q2
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----+----
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(0 rows)
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fetch backward 1 in c3;
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q1 | q2
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------------------+-------------------
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4567890123456789 | -4567890123456789
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(1 row)
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fetch backward all in c3;
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q1 | q2
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------------------+------------------
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4567890123456789 | 4567890123456789
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(1 row)
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fetch backward 1 in c3;
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q1 | q2
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----+----
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(0 rows)
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fetch all in c3;
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q1 | q2
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------------------+-------------------
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4567890123456789 | 4567890123456789
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4567890123456789 | -4567890123456789
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(2 rows)
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declare c4 cursor for select * from int8_tbl offset 10;
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fetch all in c4;
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q1 | q2
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----+----
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(0 rows)
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fetch 1 in c4;
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q1 | q2
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----+----
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(0 rows)
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fetch backward 1 in c4;
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q1 | q2
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----+----
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(0 rows)
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fetch backward all in c4;
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q1 | q2
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----+----
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(0 rows)
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fetch backward 1 in c4;
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q1 | q2
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----+----
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(0 rows)
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fetch all in c4;
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q1 | q2
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----+----
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(0 rows)
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rollback;
111290
-- Stress test for variable LIMIT in conjunction with bounded-heap sorting
112291
SELECT
113292
(SELECT n

‎src/test/regress/sql/limit.sql‎

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Original file line numberDiff line numberDiff line change
@@ -31,6 +31,48 @@ SELECT ''::text AS five, unique1, unique2, stringu1
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FROM onek
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ORDER BY unique1 LIMIT 5 OFFSET 900;
3333

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-- Test null limit and offset. The planner would discard a simple null
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-- constant, so to ensure executor is exercised, do this:
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select * from int8_tbl limit (case when random() < 0.5 then null::bigint end);
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select * from int8_tbl offset (case when random() < 0.5 then null::bigint end);
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-- Test assorted cases involving backwards fetch from a LIMIT plan node
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begin;
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declare c1 cursor for select * from int8_tbl limit 10;
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fetch all in c1;
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fetch 1 in c1;
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fetch backward 1 in c1;
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fetch backward all in c1;
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fetch backward 1 in c1;
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fetch all in c1;
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declare c2 cursor for select * from int8_tbl limit 3;
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fetch all in c2;
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fetch 1 in c2;
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fetch backward 1 in c2;
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fetch backward all in c2;
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fetch backward 1 in c2;
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fetch all in c2;
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declare c3 cursor for select * from int8_tbl offset 3;
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fetch all in c3;
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fetch 1 in c3;
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fetch backward 1 in c3;
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fetch backward all in c3;
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fetch backward 1 in c3;
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fetch all in c3;
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declare c4 cursor for select * from int8_tbl offset 10;
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fetch all in c4;
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fetch 1 in c4;
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fetch backward 1 in c4;
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fetch backward all in c4;
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fetch backward 1 in c4;
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fetch all in c4;
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rollback;
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-- Stress test for variable LIMIT in conjunction with bounded-heap sorting
3577

3678
SELECT

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