Sql 选择带索引的列或使用带索引的列非常慢
我的蟑螂数据库中有一个具有以下定义的表:Sql 选择带索引的列或使用带索引的列非常慢,sql,performance,composite-primary-key,database-indexes,cockroachdb,Sql,Performance,Composite Primary Key,Database Indexes,Cockroachdb,我的蟑螂数据库中有一个具有以下定义的表: CREATE TABLE foo_value ( foo_id_a INT NOT NULL, foo_id_b INT NOT NULL, value FLOAT NULL, create_date_time TIMESTAMP NULL, update_date_time TIMESTAMP NULL, CONSTRAINT "primary" PRIMARY KEY (foo_id_a ASC, fo
CREATE TABLE foo_value (
foo_id_a INT NOT NULL,
foo_id_b INT NOT NULL,
value FLOAT NULL,
create_date_time TIMESTAMP NULL,
update_date_time TIMESTAMP NULL,
CONSTRAINT "primary" PRIMARY KEY (foo_id_a ASC, foo_id_b ASC),
INDEX foo_value_foo_id_a_foo_id_b_idx (foo_id_a ASC, foo_id_b ASC),
INDEX foo_id_a_idx (foo_id_a ASC),
INDEX foo_id_b_idx (foo_id_b ASC),
FAMILY "primary" (foo_id_a, foo_id_b, value, create_date_time, update_date_time)
)
它包含大约400000行
查询两个ID中的一个很快:
SELECT * FROM foo_db.foo_value WHERE foo_id_a = 123456;
takes 0.071 s
SELECT * FROM foo_db.foo_value WHERE foo_id_b = 123456;
takes 0.086 s
但是,查询其中一个的速度非常慢:
SELECT * FROM foo_db.foo_value WHERE foo_id_a = 123456 OR foo_id_b = 123456;
takes 2.739 s
为什么会这样
解释的结果如下所示:
EXPLAIN SELECT * FROM foo_db.foo_value WHERE foo_id_a = 321210483;
+-------+------+-------+-----------------------+
| Level | Type | Field | Description |
+-------+------+-------+-----------------------+
| 0 | scan | | |
| 0 | | table | foo_value@primary |
| 0 | | spans | /321210483-/321210484 |
+-------+------+-------+-----------------------+
EXPLAIN SELECT * FROM foo_db.foo_value WHERE foo_id_b = 321210483;
+-------+------------+-------+------------------------+
| Level | Type | Field | Description |
+-------+------------+-------+------------------------+
| 0 | index-join | | |
| 1 | scan | | |
| 1 | | table | foo_value@foo_id_b_idx |
| 1 | | spans | /321210483-/321210484 |
| 1 | scan | | |
| 1 | | table | foo_value@primary |
+-------+------------+-------+------------------------+
EXPLAIN SELECT * FROM foo_db.foo_value WHERE foo_id_a = 321210483 OR foo_id_b = 321210483;
+-------+------+-------+-------------------+
| Level | Type | Field | Description |
+-------+------+-------+-------------------+
| 0 | scan | | |
| 0 | | table | foo_value@primary |
| 0 | | spans | ALL |
+-------+------+-------+-------------------+
您需要的是索引优化,它在or中使用两个不同的索引。遗憾的是,SQL引擎通常不支持这种优化,尽管Oracle和其他一些数据库一样支持这种优化 您最好使用union all: 注意:第二个WHERE子句可能需要考虑空值
每个子查询都将使用索引进行正确优化。建议的和实际的执行计划是什么样子的?@SimonPrice:您是指EXPLAIN VERBOSE的输出吗?
SELECT *
FROM foo_db.foo_value
WHERE foo_id_a = 123456;
UNION ALL
SELECT *
FROM foo_db.foo_value
WHERE foo_id_b = 123456 AND foo_id_a <> 123456;