在以下查询中,您将看到使用<运算符的查询仅花费0.007s,而使用>运算符的查询花费0.474s。那是巨大的区别。会发生什么事?
postgresql> SHOW server_version;
+------------------+
| server_version |
|------------------|
| 9.4.17 |
+------------------+
SHOW
Time: 0.006s
postgresql> \d msg
+------------------+-----------------------------+-------------------------------------------------------------+
| Column | Type | Modifiers |
|------------------+-----------------------------+-------------------------------------------------------------|
| id | bigint | not null default nextval('msg_id_seq'::regclass) |
| content | text | default ''::text |
... ...
| created_at | timestamp without time zone | default timezone('UTC'::text, now()) |
+------------------+-----------------------------+-------------------------------------------------------------+
Indexes:
"msg_pkey" PRIMARY KEY, btree (id)
"ix_msg_created_at" btree (created_at)
Time: 0.013s
postgresql> SELECT id FROM msg WHERE created_at < '2020-08-27' ORDER BY id desc LIMIT 1;
+--------+
| id |
|--------|
| 973604 |
+--------+
SELECT 1
Time: 0.007s
postgresql> SELECT id FROM msg WHERE created_at > '2020-08-27' ORDER BY id LIMIT 1;
+--------+
| id |
|--------|
| 973605 |
+--------+
SELECT 1
Time: 0.474s
postgresql> SELECT count(*) FROM msg WHERE created_at < '2020-08-27';
+---------+
| count |
|---------|
| 967498 |
+---------+
SELECT 1
Time: 0.509s
postgresql> SELECT count(*) FROM msg WHERE created_at > '2020-08-27';
+---------+
| count |
|---------|
| 1089 |
+---------+
SELECT 1
Time: 0.007s
postgresql> EXPLAIN ANALYZE SELECT id FROM msg WHERE created_at < '2020-08-27' ORDER BY id desc LIMIT 1;
+------------------------------------------------------------------------------------------------------------------------------------------------------------+
| QUERY PLAN |
|------------------------------------------------------------------------------------------------------------------------------------------------------------|
| Limit (cost=0.43..0.53 rows=1 width=8) (actual time=0.770..0.770 rows=1 loops=1) |
| -> Index Scan Backward using msg_pkey on msg (cost=0.43..114629.92 rows=1092141 width=8) (actual time=0.770..0.770 rows=1 loops=1) |
| Filter: (created_at < '2020-08-27 00:00:00'::timestamp without time zone) |
| Rows Removed by Filter: 1103 |
| Planning time: 0.110 ms |
| Execution time: 0.782 ms |
+------------------------------------------------------------------------------------------------------------------------------------------------------------+
EXPLAIN
Time: 0.008s
postgresql> EXPLAIN ANALYZE SELECT id FROM msg WHERE created_at > '2020-08-27' ORDER BY id LIMIT 1;
+----------------------------------------------------------------------------------------------------------------------------------------------------+
| QUERY PLAN |
|----------------------------------------------------------------------------------------------------------------------------------------------------|
| Limit (cost=0.43..44.58 rows=1 width=8) (actual time=481.168..481.169 rows=1 loops=1) |
| -> Index Scan using msg_pkey on msg (cost=0.43..114629.92 rows=2596 width=8) (actual time=481.166..481.166 rows=1 loops=1) |
| Filter: (created_at > '2020-08-27 00:00:00'::timestamp without time zone) |
| Rows Removed by Filter: 967498 |
| Planning time: 0.124 ms |
| Execution time: 481.184 ms |
+----------------------------------------------------------------------------------------------------------------------------------------------------+
EXPLAIN
Time: 0.489s
更新以添加行数(2020-09-01):
更新以添加查询计划(2020-09-01)。
msg_pkey
而不是ix_msg_created_at
。 Rows Removed by Filter
的差异可以解释性能差异。答案 0 :(得分:2)
如EXPLAIN
的输出所示,似乎两个查询都扫描索引msg_pkey
而不是ix_msg_created_at
。如图Rows Removed by Filter
所示,使用>
运算符的查询扫描967498行,而使用<
运算符的查询仅扫描1103行。这应该可以解释巨大的性能差异。
感谢所有帮助和建议。