PostgreSQL：未使用的索引导致查询性能不佳？

Question

我们有一个查询来获取在特定时间段内发生变化的所有作业。根据所选时间段，性能从一天 <100 毫秒到一周约 7 秒。

我发现如果时间段足够小，使用索引，查询速度很快。如果周期变得太大，则不使用索引并且查询变慢。

服务器运行版本 9.2。

为什么会这样，如何解决这个问题？

创建脚本：

CREATE TABLE IF NOT EXISTS "Job" 
(
    "id" serial PRIMARY KEY,
    "serial" TEXT NOT NULL
);
CREATE UNIQUE INDEX "index_Job_serial" ON "Job" ("serial" ASC);

CREATE TABLE IF NOT EXISTS "Property" 
(
    "id" serial PRIMARY KEY,
    "name" TEXT NOT NULL
);

CREATE TABLE IF NOT EXISTS "Timestamp"
(
    "id" serial PRIMARY KEY,
    "usSince1970" BIGINT NOT NULL ,
    "localTime" TEXT
);
CREATE INDEX "index_Timestamp_usSince1970" ON "Timestamp" USING btree ("usSince1970");

CREATE  TABLE IF NOT EXISTS "Changes" 
(
    "idJob" INTEGER  NOT NULL ,
    "idProperty" INTEGER  NOT NULL ,
    "idTimestamp" INTEGER  NOT NULL ,
    "value1" decimal(25,5),
    "value2" INTEGER ,
    "value3" TEXT ,
    PRIMARY KEY ("idJob", "idProperty", "idTimestamp") ,
    FOREIGN KEY ("idJob" ) REFERENCES "Job" ("id" ) ,
    FOREIGN KEY ("idProperty" ) REFERENCES "Property" ("id" ) ,
    FOREIGN KEY ("idTimestamp" ) REFERENCES "Timestamp" ("id" )
);
CREATE INDEX "index_Changes_idJob" ON "Changes" ("idJob" ASC);
CREATE INDEX "index_Changes_idProperty" ON "Changes" ("idProperty" ASC);
CREATE INDEX "index_Changes_idTimestamp" ON "Changes" ("idTimestamp" DESC);

快速查询：

-- fast query (1 day)
SELECT DISTINCT "idJob"
FROM "Changes" 
INNER JOIN "Timestamp" ON "Timestamp"."id" = "Changes"."idTimestamp" 
WHERE "Timestamp"."usSince1970" between 1584831600000000 and 1584745200000000 

-- explain
HashAggregate  (cost=26383.48..26444.33 rows=6085 width=4) (actual time=8.039..8.078 rows=179 loops=1)
  ->  Nested Loop  (cost=0.00..26368.26 rows=6085 width=4) (actual time=0.031..7.059 rows=6498 loops=1)
        ->  Index Scan using "index_Timestamp_usSince1970" on "Timestamp"  (cost=0.00..96.25 rows=2510 width=4) (actual time=0.022..0.514 rows=2671 loops=1)
              Index Cond: (("usSince1970" >= 1584745200000000::bigint) AND ("usSince1970" <= 1584831600000000::bigint))
        ->  Index Scan using "index_Changes_idTimestamp" on "Changes"  (cost=0.00..10.27 rows=20 width=8) (actual time=0.002..0.002 rows=2 loops=2671)
              Index Cond: ("idTimestamp" = "Timestamp".id)
Total runtime: 8.204 ms

慢查询：

-- slow query (7 days)
SELECT distinct "idJob"
FROM "Changes" 
INNER JOIN "Timestamp" ON "Timestamp"."id" = "Changes"."idTimestamp" 
WHERE "Timestamp"."usSince1970" between 1583708400000000 and 1584313200000000

-- explain
Unique  (cost=570694.82..571824.16 rows=92521 width=4) (actual time=8869.569..8930.545 rows=3695 loops=1)
  ->  Sort  (cost=570694.82..571259.49 rows=225867 width=4) (actual time=8869.568..8915.372 rows=260705 loops=1)
        Sort Key: "Changes"."idJob"
        Sort Method: external merge  Disk: 3552kB
        ->  Hash Join  (cost=4926.44..547518.97 rows=225867 width=4) (actual time=6325.494..8734.353 rows=260705 loops=1)
              Hash Cond: ("Changes"."idTimestamp" = "Timestamp".id)
              ->  Seq Scan on "Changes"  (cost=0.00..250722.43 rows=16238343 width=8) (actual time=0.004..2505.794 rows=16238343 loops=1)
              ->  Hash  (cost=3397.79..3397.79 rows=93172 width=4) (actual time=42.392..42.392 rows=107093 loops=1)
                    Buckets: 4096  Batches: 4  Memory Usage: 948kB
                    ->  Index Scan using "index_Timestamp_usSince1970" on "Timestamp"  (cost=0.00..3397.79 rows=93172 width=4) (actual time=0.006..20.831 rows=107093 loops=1)
                          Index Cond: (("usSince1970" >= 1583708400000000::bigint) AND ("usSince1970" <= 1584313200000000::bigint))
Total runtime: 8932.374 ms

提前致谢。

Answer 1

慢速查询处理更多的数据（100000 对来自 "Timestamp" 的 2500 行），所以速度慢也就不足为奇了。

您也可以强制 PostgreSQL 对慢速查询使用嵌套循环连接：

BEGIN;
SET LOCAL enable_hashjoin = off;
SET LOCAL enable_mergejoin = off;
SELECT ...;
COMMIT;

尝试一下，看看 PostgreSQL 是否正确，散列连接是否真的很慢。

我怀疑 PostgreSQL 在这里做的是正确的，提高性能的最佳方法是增加 work_mem。

如果您愿意添加另一个索引并经常添加到 VACUUM "Changes"，您可以通过仅索引扫描获得更好的性能：

CREATE INDEX ON "Changes" ("idTimestamp") INCLUDE ("idJob");

在旧版本的 PostgreSQL 上会是

CREATE INDEX ON "Changes" ("idTimestamp", "idJob");

那么你最好删除现在不需要的索引 "index_Changes_idTimestamp"。

顺便说一句，您使用驼峰式大小写和引号引起来的标识符让您的生活变得不必要地困难。

Answer 2

顺便说一句：您的查询等同于：

SELECT DISTINCT "idJob"
FROM "Changes" ch
WHERE EXISTS ( 
        SELECT * FROM "Timestamp"  ts
        WHERE ts.id = ch."idTimestamp" 
        AND ts."usSince1970" between 1584831600000000 and 1584745200000000 
        );

PostgreSQL：未使用的索引导致查询性能不佳？

PostgreSQL: Bad query performance caused by unused index?

sql

postgresql

database-performance

postgresql-9.2