Sql 为什么常量的类型转换会破坏查询性能

测试表和索引（PostgreSQL 12.1）：

在第一个查询中，一切正常，使用适当的索引“ind”：

为什么使用“2020-02-08”（已经是文本）的“：：text”类型转换查询性能非常低

explain (analyze, buffers)
SELECT *
FROM t
WHERE
   ('2020-02-08'::text)::date IS NULL
   OR
   dt > '2020-02-08'
ORDER BY dt
LIMIT 1

"Limit  (cost=0.29..0.44 rows=1 width=8) (actual time=45.306..45.307 rows=1 loops=1)"
"  Buffers: shared hit=6232"
"  ->  Index Only Scan using ind on t  (cost=0.29..561.28 rows=3658 width=8) (actual time=45.304..45.304 rows=1 loops=1)"
"        Filter: ((('2020-02-08'::cstring)::date IS NULL) OR (dt > '2020-02-08 00:00:00+05'::timestamp with time zone))"
"        Rows Removed by Filter: 6367"
"        Heap Fetches: 6368"
"        Buffers: shared hit=6232"
"Planning Time: 0.348 ms"
"Execution Time: 45.343 ms"

---

非空常量何时为空？看起来pg意识到它不是在第一种情况下，并且省略了过滤器中的所有数据类型转换，但是在第二种情况下它没有（因为双重转换？），因此进行了大量的数据转换，这将引入一些额外的工作。您认为可以接受哪种毫秒的速度减慢？@CaiusJard如果用int列而不是timestamp重写整个示例，那么两个查询都能正常工作，所以问题不是双重转换-：：text:：int双重转换工作正常。我认为您犯了逻辑错误。您认为双转换：：text：：int可以很好地工作，因此：：text：：date也可以。但这些转换并不相同。

explain (analyze, buffers)
SELECT *
FROM t
WHERE
   ('2020-02-08')::date IS NULL
   OR
   dt > '2020-02-08'
ORDER BY dt
LIMIT 1

"Limit  (cost=0.29..0.37 rows=1 width=8) (actual time=0.186..0.188 rows=1 loops=1)"
"  Buffers: shared hit=3"
"  ->  Index Only Scan using ind on t  (cost=0.29..303.75 rows=3627 width=8) (actual time=0.184..0.184 rows=1 loops=1)"
"        Index Cond: (dt > '2020-02-08 00:00:00+05'::timestamp with time zone)"
"        Heap Fetches: 1"
"        Buffers: shared hit=3"
"Planning Time: 2.365 ms"
"Execution Time: 0.239 ms"

explain (analyze, buffers)
SELECT *
FROM t
WHERE
   ('2020-02-08'::text)::date IS NULL
   OR
   dt > '2020-02-08'
ORDER BY dt
LIMIT 1

"Limit  (cost=0.29..0.44 rows=1 width=8) (actual time=45.306..45.307 rows=1 loops=1)"
"  Buffers: shared hit=6232"
"  ->  Index Only Scan using ind on t  (cost=0.29..561.28 rows=3658 width=8) (actual time=45.304..45.304 rows=1 loops=1)"
"        Filter: ((('2020-02-08'::cstring)::date IS NULL) OR (dt > '2020-02-08 00:00:00+05'::timestamp with time zone))"
"        Rows Removed by Filter: 6367"
"        Heap Fetches: 6368"
"        Buffers: shared hit=6232"
"Planning Time: 0.348 ms"
"Execution Time: 45.343 ms"