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测试表和索引（PostgreSQL 12.1）：

CREATE TABLE t (dt timestamp with time zone);
CREATE INDEX ind ON t USING btree (dt);

INSERT
INTO t(dt)
    SELECT
    (
        timestamp '2020-01-01 00:00:00' +
        random() * (
            timestamp '2020-02-29 00:00:00' -
            timestamp '2020-01-01 00:00:00'
        )
    )
    FROM generate_series(1, 10000)

在第一个查询中，一切正常，使用了适当的索引“ind”：

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"

为什么使用' ::text '类型转换'2020-02-08'（已经是文本）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"
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