Question

我有一个包含许多行（数百万）的大表，其中包含JSONB / HSTORE类型的列，其中包含许多字段（数百个）。为了说明，我使用下面较小且不太复杂的表：

-- table with HSTORE column
CREATE TABLE test_hstore (id BIGSERIAL PRIMARY KEY, data HSTORE);
INSERT INTO test_hstore (data)
SELECT hstore(
    '  key_1=>' || trunc(2 * random()) ||
    ', key_2=>' || trunc(2 * random()) ||
    ', key_3=>' || trunc(2 * random()))
FROM generate_series(0, 9999999) i;

-- table with JSONB column
CREATE TABLE test_jsonb (id BIGSERIAL PRIMARY KEY, data JSONB);
INSERT INTO test_jsonb (data)
SELECT (
    '{ "key_1":' || trunc(2 * random()) ||
    ', "key_2":' || trunc(2 * random()) ||
    ', "key_3":' || trunc(2 * random()) || '}')::JSONB
FROM generate_series(0, 9999999) i;

我想在SELECT列中data一个或多个字段而不使用WHERE子句。随着选定字段数量的增加，性能会下降：

EXPLAIN ANALYSE
SELECT id FROM test_hstore;
--Seq Scan on test_hstore  (cost=0.00..213637.56 rows=10000056 width=8) (actual time=0.049..3705.852 rows=10000000 loops=1)
--Planning time: 0.419 ms
--Execution time: 5445.654 ms

EXPLAIN ANALYSE
SELECT data FROM test_hstore;
--Seq Scan on test_hstore  (cost=0.00..213637.56 rows=10000056 width=56) (actual time=0.083..2424.334 rows=10000000 loops=1)
--Planning time: 0.082 ms
--Execution time: 3856.972 ms

EXPLAIN ANALYSE
SELECT data->'key_1' FROM test_hstore;
--Seq Scan on test_hstore  (cost=0.00..238637.70 rows=10000056 width=32) (actual time=0.122..3263.937 rows=10000000 loops=1)
--Planning time: 0.052 ms
--Execution time: 5390.803 ms


EXPLAIN ANALYSE
SELECT data->'key_1', data->'key_2' FROM test_hstore;
--Seq Scan on test_hstore  (cost=0.00..263637.84 rows=10000056 width=64) (actual time=0.089..3621.768 rows=10000000 loops=1)
--Planning time: 0.051 ms
--Execution time: 5334.452 ms

EXPLAIN ANALYSE
SELECT data->'key_1', data->'key_2', data->'key_3' FROM test_hstore;
--Seq Scan on test_hstore  (cost=0.00..288637.98 rows=10000056 width=96) (actual time=0.086..4291.111 rows=10000000 loops=1)
--Planning time: 0.067 ms
--Execution time: 6375.229 ms

JSONB列类型的相同趋势（甚至更明显）：

EXPLAIN ANALYSE
SELECT id FROM test_jsonb;
--Seq Scan on test_jsonb  (cost=0.00..233332.28 rows=9999828 width=8) (actual time=0.028..4009.841 rows=10000000 loops=1)
--Planning time: 0.878 ms
--Execution time: 5867.604 ms

EXPLAIN ANALYSE
SELECT data FROM test_jsonb;
--Seq Scan on test_jsonb  (cost=0.00..233332.28 rows=9999828 width=68) (actual time=0.074..2371.212 rows=10000000 loops=1)
--Planning time: 0.061 ms
--Execution time: 3787.308 ms

EXPLAIN ANALYSE
SELECT data->'key_1' FROM test_jsonb;
--Seq Scan on test_jsonb  (cost=0.00..258331.85 rows=9999828 width=32) (actual time=0.106..4677.026 rows=10000000 loops=1)
--Planning time: 0.066 ms
--Execution time: 6382.469 ms

EXPLAIN ANALYSE
SELECT data->'key_1', data->'key_2' FROM test_jsonb;
--Seq Scan on test_jsonb  (cost=0.00..283331.42 rows=9999828 width=64) (actual time=0.094..6888.904 rows=10000000 loops=1)
--Planning time: 0.047 ms
--Execution time: 8593.060 ms

EXPLAIN ANALYSE
SELECT data->'key_1', data->'key_2', data->'key_3' FROM test_jsonb;
--Seq Scan on test_jsonb  (cost=0.00..308330.99 rows=9999828 width=96) (actual time=0.173..9567.699 rows=10000000 loops=1)
--Planning time: 0.171 ms
--Execution time: 11262.135 ms

当表包含更多字段时，这变得更加明显。有解决方法吗？

添加GIN INDEX似乎没有帮助：

CREATE INDEX ix_test_hstore ON test_hstore USING GIN (data);
EXPLAIN ANALYSE
SELECT data->'key_1', data->'key_2', data->'key_3' FROM test_hstore;
--Seq Scan on test_hstore  (cost=0.00..288637.00 rows=10000000 width=96) (actual time=0.045..4650.447 rows=10000000 loops=1)
--Planning time: 2.100 ms
--Execution time: 6746.631 ms

CREATE INDEX ix_test_jsonb ON test_jsonb USING GIN (data);
EXPLAIN ANALYSE
SELECT data->'key_1', data->'key_2', data->'key_3' FROM test_jsonb;
--Seq Scan on test_jsonb  (cost=0.00..308334.00 rows=10000000 width=96) (actual time=0.149..9807.012 rows=10000000 loops=1)
--Planning time: 0.131 ms
--Execution time: 11739.948 ms

Answer 1

实际上，您无法在数据存储中改进对key的访问权限，或者JSON数据的property（可能是数组，或字符串或数字;这可能是为什么检索它比从hstore检索它更困难的原因。

如果您需要在WHERE子句中使用data->key_1，索引可以帮助您，但它不会使从数据中检索属性变得更容易。

如果您总是（或经常）使用某个key_1，那么最好的行动方案就是规范化您的数据并命名列 key_1。如果您的数据源使您可以轻松存储data，但不容易存储key_1，那么您可以使用触发功能（插入或更新时））从column key_1：
的值填充data
CREATE TABLE test_jsonb ( id BIGSERIAL PRIMARY KEY, data JSONB, key_1 integer ); CREATE OR REPLACE FUNCTION ins_upd_test_data() RETURNS trigger AS $$ BEGIN new.key_1 = (new.data->>'key_1')::integer ; RETURN new ; END ; $$ LANGUAGE plpgsql VOLATILE LEAKPROOF; CREATE TRIGGER ins_upd_test_jsonb_trigger BEFORE INSERT OR UPDATE OF data ON test_jsonb FOR EACH ROW EXECUTE PROCEDURE ins_upd_test_data();

通过这种方式，您可以检索key_1，效率与检索id相同。

PostgreSQL：有没有办法使用JSONB或HSTORE密钥提高SELECT查询的性能？

1 个答案: