Медленный запрос JOIN с OR в операторе WHERE
Вот простой пример моей проблемы:
CREATE TABLE test1 (id SERIAL, key TEXT UNIQUE, value TEXT);
CREATE TABLE test2 (id SERIAL, key TEXT UNIQUE, value TEXT);
INSERT INTO test1 (key, value)
SELECT i::TEXT, 'ABC' || i::TEXT
FROM generate_series(0, 1000000) AS i;
INSERT INTO test2 (key, value)
SELECT i::TEXT, 'ABC' || (i+1000)::TEXT
FROM generate_series(0, 600000) AS i;
INSERT INTO test2 (key, value)
SELECT i::TEXT, 'ABC' || (i+1000)::TEXT
FROM generate_series(1000000, 1200000) AS i;
CREATE INDEX test1_key ON test1 (key);
CREATE INDEX test1_value ON test1 (value);
CREATE INDEX test2_key ON test2 (key);
CREATE INDEX test2_value ON test2 (value);
VACUUM FULL VERBOSE ANALYZE test1;
VACUUM FULL VERBOSE ANALYZE test2;
Это тот запрос, который я сейчас использую, но он занимает более 6 секунд.
EXPLAIN ANALYZE
SELECT test1.key AS key1, test1.value AS value1,
test2.key AS key2, test2.value AS value2
FROM test1
LEFT OUTER JOIN test2 ON (test1.key = test2.key)
WHERE test1.value = 'ABC1234' OR test2.value = 'ABC1234';
key1 | value1 | key2 | value2
------+---------+------+---------
234 | ABC234 | 234 | ABC1234
1234 | ABC1234 | 1234 | ABC2234
(2 rows)
QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------
Hash Left Join (cost=27344.05..79728.10 rows=2 width=32) (actual time=5428.635..6097.098 rows=2 loops=1)
Hash Cond: (test1.key = test2.key)
Filter: ((test1.value = 'ABC1234'::text) OR (test2.value = 'ABC1234'::text))
-> Seq Scan on test1 (cost=0.00..16321.01 rows=1000001 width=15) (actual time=0.009..1057.315 rows=1000001 loops=1)
-> Hash (cost=13047.02..13047.02 rows=800002 width=17) (actual time=2231.964..2231.964 rows=800002 loops=1)
Buckets: 65536 Batches: 2 Memory Usage: 14551kB
-> Seq Scan on test2 (cost=0.00..13047.02 rows=800002 width=17) (actual time=0.010..980.232 rows=800002 loops=1)
Total runtime: 6109.042 ms
(8 rows)
В обеих таблицах только очень немногие наборы данных будут соответствовать требованиям, но, похоже, этот факт не соблюдается. Вместо этого я могу использовать такой запрос:
EXPLAIN ANALYZE
SELECT coalesce(test1.key, test3.key1) AS key1, coalesce(test1.value, test3.value1) AS value1,
coalesce(test2.key, test3.key2) AS key2, coalesce(test2.value, test3.value2) AS value2
FROM (SELECT test1.key AS key1, test1.value AS value1,
test2.key AS key2, test2.value AS value2
FROM (SELECT key, value FROM test1 WHERE value = 'ABC1234') AS test1
FULL JOIN (SELECT key, value FROM test2 WHERE value = 'ABC1234') AS test2
ON (test1.key = test2.key)) AS test3
LEFT OUTER JOIN test1 ON (test1.key = test3.key2)
LEFT OUTER JOIN test2 ON (test2.key = test3.key1)
WHERE test1.key IS NOT NULL;
key1 | value1 | key2 | value2
------+---------+------+---------
1234 | ABC1234 | 1234 | ABC2234
234 | ABC234 | 234 | ABC1234
(2 rows)
QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------------
Nested Loop Left Join (cost=0.00..33.56 rows=1 width=64) (actual time=0.075..0.083 rows=1 loops=1)
-> Nested Loop (cost=0.00..25.19 rows=1 width=47) (actual time=0.066..0.072 rows=1 loops=1)
-> Nested Loop Left Join (cost=0.00..16.80 rows=1 width=32) (actual time=0.051..0.054 rows=1 loops=1)
-> Index Scan using test2_value_key on test2 (cost=0.00..8.41 rows=1 width=17) (actual time=0.026..0.027 rows=1 loops=1)
Index Cond: (value = 'ABC1234'::text)
-> Index Scan using test1_key on test1 (cost=0.00..8.38 rows=1 width=15) (actual time=0.020..0.020 rows=0 loops=1)
Index Cond: (public.test1.key = public.test2.key)
Filter: (public.test1.value = 'ABC1234'::text)
-> Index Scan using test1_key on test1 (cost=0.00..8.38 rows=1 width=15) (actual time=0.011..0.013 rows=1 loops=1)
Index Cond: ((public.test1.key IS NOT NULL) AND (public.test1.key = public.test2.key))
-> Index Scan using test2_key on test2 (cost=0.00..8.36 rows=1 width=17) (actual time=0.001..0.001 rows=0 loops=1)
Index Cond: (public.test2.key = public.test1.key)
Total runtime: 0.139 ms
Следующий запрос проще, но все же слишком медленный:
EXPLAIN ANALYZE
SELECT test1.key AS key1, test1.value AS value1,
test2.key AS key2, test2.value AS value2
FROM test1
LEFT OUTER JOIN test2 ON (test1.key = test2.key)
WHERE test1.value = 'ABC1234'
OR EXISTS (SELECT 1 FROM test2 t WHERE t.key = test1.key AND t.value = 'ABC1234');
key1 | value1 | key2 | value2
------+---------+------+---------
1234 | ABC1234 | 1234 | ABC2234
234 | ABC234 | 234 | ABC1234
(2 rows)
QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------------------
Merge Left Join (cost=0.00..8446826.32 rows=500001 width=32) (actual time=615.706..1651.370 rows=2 loops=1)
Merge Cond: (test1.key = test2.key)
-> Index Scan using test1_key on test1 (cost=0.00..8398983.25 rows=500001 width=15) (actual time=28.449..734.567 rows=2 loops=1)
Filter: ((value = 'ABC1234'::text) OR (alternatives: SubPlan 1 or hashed SubPlan 2))
SubPlan 1
-> Index Scan using test2_key on test2 t (cost=0.00..8.36 rows=1 width=0) (never executed)
Index Cond: (key = $0)
Filter: (value = 'ABC1234'::text)
SubPlan 2
-> Index Scan using test2_value on test2 t (cost=0.00..8.37 rows=1 width=7) (actual time=0.376..0.380 rows=1 loops=1)
Index Cond: (value = 'ABC1234'::text)
-> Index Scan using test2_key on test2 (cost=0.00..39593.05 rows=800002 width=17) (actual time=0.019..498.456 rows=348894 loops=1)
Total runtime: 1651.453 ms
(13 rows)
Итак, мой вопрос: существует ли простой запрос, который приведет к такому же плану быстрого выполнения, как второй запрос или может быть, указатель или какая-то подсказка для планировщика.
(Я знаю, что для этого примера было бы разумно иметь только одну таблицу с обоими значениями в ней. Но на самом деле таблицы более сложные, и схема таблиц не может можно легко изменить.)
PostgreSQL Version: 9.0.3
shared_buffers = 64MB
effective_cache_size = 32MB
work_mem = 16MB
maintenance_work_mem = 32MB
temp_buffers = 8MB
wal_buffers= 1MB
РЕДАКТИРОВАТЬ: Как предложил Кипотлов, вот версия UNION. Почему обычный запрос OR не выбирает такой хороший план?
EXPLAIN ANALYZE
SELECT test1.key AS key1, test1.value AS value1,
test2.key AS key2, test2.value AS value2
FROM test1
LEFT OUTER JOIN test2 ON (test1.key = test2.key)
WHERE test1.value = 'ABC1234'
UNION
SELECT test1.key AS key1, test1.value AS value1,
test2.key AS key2, test2.value AS value2
FROM test1
LEFT OUTER JOIN test2 ON (test1.key = test2.key)
WHERE test2.value = 'ABC1234';
key1 | value1 | key2 | value2
------+---------+------+---------
1234 | ABC1234 | 1234 | ABC2234
234 | ABC234 | 234 | ABC1234
(2 rows)
QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------------------
Unique (cost=33.64..33.66 rows=2 width=32) (actual time=0.114..0.119 rows=2 loops=1)
-> Sort (cost=33.64..33.64 rows=2 width=32) (actual time=0.111..0.113 rows=2 loops=1)
Sort Key: public.test1.key, public.test1.value, public.test2.key, public.test2.value
Sort Method: quicksort Memory: 17kB
-> Append (cost=0.00..33.63 rows=2 width=32) (actual time=0.046..0.097 rows=2 loops=1)
-> Nested Loop Left Join (cost=0.00..16.81 rows=1 width=32) (actual time=0.044..0.050 rows=1 loops=1)
-> Index Scan using test1_value_key on test1 (cost=0.00..8.44 rows=1 width=15) (actual time=0.023..0.024 rows=1 loops=1)
Index Cond: (value = 'ABC1234'::text)
-> Index Scan using test2_key on test2 (cost=0.00..8.36 rows=1 width=17) (actual time=0.014..0.016 rows=1 loops=1)
Index Cond: (public.test1.key = public.test2.key)
-> Nested Loop (cost=0.00..16.80 rows=1 width=32) (actual time=0.036..0.041 rows=1 loops=1)
-> Index Scan using test2_value_key on test2 (cost=0.00..8.41 rows=1 width=17) (actual time=0.019..0.020 rows=1 loops=1)
Index Cond: (value = 'ABC1234'::text)
-> Index Scan using test1_key on test1 (cost=0.00..8.38 rows=1 width=15) (actual time=0.013..0.015 rows=1 loops=1)
Index Cond: (public.test1.key = public.test2.key)
Total runtime: 0.173 ms
(16 rows)
7
задан rudi-moore 18 March 2011 в 15:19
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