使用索引改进使用 Seq Scan 的查询

Question

我正在尝试优化此查询。 它正在执行顺序扫描，它与行数呈线性增长。

SELECT "class"."starts_at"
FROM "class"
WHERE ("class"."starts_at" >= 2021-12-16 14:13:19.824533+00:00
       AND "class"."starts_at" BETWEEN 2021-12-01 00:00:00+05:00 AND 2021-12-31 23:59:59+05:00
       AND "class"."status" = reserved
       AND "class"."teacher_id" = 3)

Query PlanSeq Scan on class  (cost=0.00..1.18 rows=1 width=8) (actual time=0.029..0.056 rows=1 loops=1)
  Filter: ((starts_at >= '2021-12-16 14:13:19.824533+00'::timestamp with time zone) AND (starts_at >= '2021-11-30 19:00:00+00'::timestamp with time zone) AND (starts_at <= '2021-12-31 18:59:59+00'::timestamp with time zone) AND ((status)::text = 'reserved'::text) AND (teacher_id = 3))
  Rows Removed by Filter: 7
Planning Time: 0.138 ms
Execution Time: 0.138 ms

当我为starts_at 、 status和teacher添加索引时，它没有使用索引。

BEGIN;
--
-- Create index starts_at_status_teacher_idx on field(s) starts_at, status, teacher of model class
--
CREATE INDEX "starts_at_status_teacher_idx" ON "class" ("starts_at", "status", "teacher_id");
COMMIT;

我需要添加什么索引以使其更快？

Answer 1

您的索引应该是：

CREATE INDEX X0001 ON "class" (teacher_id, status, starts_at);

当谓词中存在不等式和相等性时，必须首先在索引键中设置相等的列，最后设置不等式列...

如果您在 Microsoft SQL 服务器中执行了此查询，优化器会按照键中列的正确顺序为您提供正确的索引...