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134 lines
4.9 KiB
SQL
134 lines
4.9 KiB
SQL
/* Aggregate function 'uniq' is intended to be associative and provide deterministic results regardless to the schedule of query execution threads and remote servers in a cluster.
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* But due to subtle bug in implementation it is not associative in very rare cases.
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* In this test we fill data structure with specific pattern that reproduces this behaviour.
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*/
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DROP TABLE IF EXISTS part_a;
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DROP TABLE IF EXISTS part_b;
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DROP TABLE IF EXISTS part_c;
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DROP TABLE IF EXISTS part_d;
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/* Create values that will resize hash table to the maximum (131072 cells) and fill it with less than max_fill (65536 cells)
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* and occupy cells near the end except last 10 cells:
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* [ ----------- ]
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* Pick values that will vanish if table will be rehashed.
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*/
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CREATE TABLE part_a ENGINE = TinyLog AS SELECT * FROM
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(
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WITH
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number AS k1,
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bitXor(k1, bitShiftRight(k1, 33)) AS k2,
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k2 * 0xff51afd7ed558ccd AS k3,
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bitXor(k3, bitShiftRight(k3, 33)) AS k4,
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k4 * 0xc4ceb9fe1a85ec53 AS k5,
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bitXor(k5, bitShiftRight(k5, 33)) AS k6,
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k6 AS hash,
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bitShiftRight(hash, 15) % 0x20000 AS place,
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hash % 2 = 0 AS will_remain
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SELECT hash, number, place FROM system.numbers WHERE place >= 90000 AND place < 131062 AND NOT will_remain LIMIT 1 BY place LIMIT 41062
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) ORDER BY place;
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/* Create values that will resize hash table to the maximum (131072 cells) and fill it with less than max_fill (65536 cells),
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* but if we use both "a" and "b", it will force rehash.
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* [ ----------- ]
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* Pick values that will remain after rehash.
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*/
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CREATE TABLE part_b ENGINE = TinyLog AS SELECT * FROM
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(
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WITH
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number AS k1,
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bitXor(k1, bitShiftRight(k1, 33)) AS k2,
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k2 * 0xff51afd7ed558ccd AS k3,
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bitXor(k3, bitShiftRight(k3, 33)) AS k4,
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k4 * 0xc4ceb9fe1a85ec53 AS k5,
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bitXor(k5, bitShiftRight(k5, 33)) AS k6,
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k6 AS hash,
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bitShiftRight(hash, 15) % 0x20000 AS place,
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hash % 2 = 0 AS will_remain
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SELECT hash, number, place FROM system.numbers WHERE place >= 50000 AND place < 90000 AND will_remain LIMIT 1 BY place LIMIT 40000
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) ORDER BY place;
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/* Occupy 10 cells near the end of "a":
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* a: [ ----------- ]
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* c: [ -- ]
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* If we insert "a" then "c", these values will be placed at the end of hash table due to collision resolution:
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* a + c: [ aaaaaaaaaaacc]
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*/
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CREATE TABLE part_c ENGINE = TinyLog AS SELECT * FROM
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(
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WITH
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number AS k1,
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bitXor(k1, bitShiftRight(k1, 33)) AS k2,
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k2 * 0xff51afd7ed558ccd AS k3,
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bitXor(k3, bitShiftRight(k3, 33)) AS k4,
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k4 * 0xc4ceb9fe1a85ec53 AS k5,
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bitXor(k5, bitShiftRight(k5, 33)) AS k6,
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k6 AS hash,
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bitShiftRight(hash, 15) % 0x20000 AS place,
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hash % 2 = 0 AS will_remain
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SELECT hash, number, place FROM system.numbers WHERE place >= 131052 AND place < 131062 AND will_remain AND hash NOT IN (SELECT hash FROM part_a) LIMIT 1 BY place LIMIT 10
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) ORDER BY place;
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/* Occupy 10 cells at the end of hash table, after "a":
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* a: [ ----------- ]
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* d: [ --]
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* a + d: [ aaaaaaaaaaadd]
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* But if we insert "a" then "c" then "d", these values will be placed at the beginning of the hash table due to collision resolution:
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* a+c+d: [dd aaaaaaaaaaacc]
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*/
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CREATE TABLE part_d ENGINE = TinyLog AS SELECT * FROM
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(
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WITH
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number AS k1,
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bitXor(k1, bitShiftRight(k1, 33)) AS k2,
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k2 * 0xff51afd7ed558ccd AS k3,
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bitXor(k3, bitShiftRight(k3, 33)) AS k4,
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k4 * 0xc4ceb9fe1a85ec53 AS k5,
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bitXor(k5, bitShiftRight(k5, 33)) AS k6,
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k6 AS hash,
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bitShiftRight(hash, 15) % 0x20000 AS place,
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hash % 2 = 0 AS will_remain
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SELECT hash, number, place FROM system.numbers WHERE place >= 131062 AND will_remain LIMIT 1 BY place LIMIT 10
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) ORDER BY place;
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/** What happens if we insert a then c then d then b?
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* Insertion of b forces rehash.
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* a will be removed, but c, d, b remain:
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* [dd bbbbbbbbbb cc]
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* Then we go through hash table and move elements to better places in collision resolution chain.
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* c will be moved left to their right place:
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* [dd bbbbbbbbbb cc ]
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*
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* And d must be moved also:
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* [ bbbbbbbbbb ccdd]
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* But our algorithm was incorrect and it doesn't happen.
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*
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* If we insert d again, it will be placed twice because original d will not found:
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* [dd bbbbbbbbbb ccdd]
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* This will lead to slightly higher return value of "uniq" aggregate function and it is dependent on insertion order.
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*/
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SET max_threads = 1;
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/** Results of these two queries must match: */
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SELECT uniq(number) FROM (
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SELECT * FROM part_a
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UNION ALL SELECT * FROM part_c
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UNION ALL SELECT * FROM part_d
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UNION ALL SELECT * FROM part_b);
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SELECT uniq(number) FROM (
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SELECT * FROM part_a
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UNION ALL SELECT * FROM part_c
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UNION ALL SELECT * FROM part_d
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UNION ALL SELECT * FROM part_b
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UNION ALL SELECT * FROM part_d);
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DROP TABLE part_a;
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DROP TABLE part_b;
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DROP TABLE part_c;
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DROP TABLE part_d;
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