sql - 使用 && 加入 TSTZRANGE 时，Postgresql 9.4 查询变得越来越慢

Question

我正在运行的查询随着记录的添加而逐渐变慢。 通过自动化过程(bash 调用 psql)不断添加记录。我想纠正这个瓶颈；但是，我不知道我最好的选择是什么。

这是 pgBadger 的输出:

Hour    Count   Duration    Avg duration
00      9,990   10m3s       60ms     <---ignore this hour
02      1       60ms        60ms     <---ignore this hour
03      4,638   1m54s       24ms     <---queries begin with table empty
04      30,991  55m49s      108ms    <---first full hour of queries running
05      13,497  58m3s       258ms
06      9,904   58m32s      354ms
07      10,542  58m25s      332ms
08      8,599   58m42s      409ms
09      7,360   58m52s      479ms
10      6,661   58m57s      531ms
11      6,133   59m2s       577ms
12      5,601   59m6s       633ms
13      5,327   59m9s       666ms
14      4,964   59m12s      715ms
15      4,759   59m14s      746ms
16      4,531   59m17s      785ms
17      4,330   59m18s      821ms
18      939     13m16s      848ms

表结构如下:

CREATE TABLE "Parent" (
    "ParentID" SERIAL PRIMARY KEY,
    "Details1" VARCHAR
);

表 "Parent" 与表 "Foo" 具有一对多关系:

CREATE TABLE "Foo" (
    "FooID" SERIAL PRIMARY KEY,
    "ParentID" int4 NOT NULL REFERENCES "Parent" ("ParentID"),
    "Details1" VARCHAR
);

表 "Foo" 与表 "Bar" 具有一对多关系:

CREATE TABLE "Bar" (
    "FooID" int8 NOT NULL REFERENCES "Foo" ("FooID"),
    "Timerange" tstzrange NOT NULL,
    "Detail1" VARCHAR,
    "Detail2" VARCHAR,
    CONSTRAINT "Bar_pkey" PRIMARY KEY ("FooID", "Timerange")
);
CREATE INDEX  "Bar_FooID_Timerange_idx" ON "Bar" USING gist("FooID", "Timerange");

此外，表 “Bar” 不能包含相同 “FooID” 或 的重叠 “Timespan” 值>“ParentID”。 我创建了一个触发器，它在任何 INSERT、UPDATE 或 DELETE 之后触发防止重叠范围。

触发器包含一个部分，看起来类似:

WITH
    "cte" AS (
        SELECT
            "Foo"."FooID",
            "Foo"."ParentID",
            "Foo"."Details1",
            "Bar"."Timespan"
        FROM
            "Foo"
            JOIN "Bar" ON "Foo"."FooID" = "Bar"."FooID"
        WHERE
            "Foo"."FooID" = 1234
    )
SELECT
    "Foo"."FooID",
    "Foo"."ParentID",
    "Foo"."Details1",
    "Bar"."Timespan"
FROM
    "cte"
    JOIN "Foo" ON 
        "cte"."ParentID" = "Foo"."ParentID"
        AND "cte"."FooID" <> "Foo"."FooID"
    JOIN "Bar" ON
        "Foo"."FooID" = "Bar"."FooID"
        AND "cte"."Timespan" && "Bar"."Timespan";

EXPLAIN ANALYSE 的结果:

Nested Loop  (cost=7258.08..15540.26 rows=1 width=130) (actual time=8.052..147.792 rows=1 loops=1)
  Join Filter: ((cte."FooID" <> "Foo"."FooID") AND (cte."ParentID" = "Foo"."ParentID"))
  Rows Removed by Join Filter: 76
  CTE cte
    ->  Nested Loop  (cost=0.68..7257.25 rows=1000 width=160) (actual time=1.727..1.735 rows=1 loops=1)
          ->  Function Scan on "fn_Bar"  (cost=0.25..10.25 rows=1000 width=104) (actual time=1.699..1.701 rows=1 loops=1)
          ->  Index Scan using "Foo_pkey" on "Foo" "Foo_1"  (cost=0.42..7.24 rows=1 width=64) (actual time=0.023..0.025 rows=1 loops=1)
                Index Cond: ("FooID" = "fn_Bar"."FooID")
  ->  Nested Loop  (cost=0.41..8256.00 rows=50 width=86) (actual time=1.828..147.188 rows=77 loops=1)
        ->  CTE Scan on cte  (cost=0.00..20.00 rows=1000 width=108) (actual time=1.730..1.740 rows=1 loops=1)
   **** ->  Index Scan using "Bar_FooID_Timerange_idx" on "Bar"  (cost=0.41..8.23 rows=1 width=74) (actual time=0.093..145.314 rows=77 loops=1)
              Index Cond: ((cte."Timespan" && "Timespan"))
  ->  Index Scan using "Foo_pkey" on "Foo"  (cost=0.42..0.53 rows=1 width=64) (actual time=0.004..0.005 rows=1 loops=77)
        Index Cond: ("FooID" = "Bar"."FooID")
Planning time: 1.490 ms
Execution time: 147.869 ms

(**** 强调我的)

这似乎表明 99% 的工作是在从 "cte" 到 "Bar" 的 JOIN 中完成的(通过"Foo") ...但它已经在使用适当的索引...它仍然太慢了。

所以我跑了:

SELECT 
    pg_size_pretty(pg_relation_size('"Bar"')) AS "Table",
    pg_size_pretty(pg_relation_size('"Bar_FooID_Timerange_idx"')) AS "Index";

结果:

    Table    |    Index
-------------|-------------
 283 MB      | 90 MB

这种大小(相对于表)的索引在读取性能方面是否提供很多？我正在考虑一个 sudo 分区，其中索引被几个部分索引替换......也许部分需要更少的维护(和读取)并且性能会提高。我从未见过这样做，只是一个想法。如果这是一个选项，我想不出任何好的方法来限制段，因为这将在 TSTZRANGE 值上。

我还认为将 "ParentID" 添加到 "Bar" 会加快处理速度，但我不想反规范化。

我还有什么选择？

---

Erwin Brandstetter 推荐的更改的影响

在性能高峰期(18:00 点)，该过程一直每秒添加 14.5 条记录...从每秒 1.15 条记录增加。

这是结果:

1. 将 "ParentID" 添加到表 "Bar"
2. 向"Foo"("ParentID", "FooID")添加外键约束
3. 添加 EXCLUDE USING gist ("ParentID"WITH =, "Timerange"WITH &&) DEFERRABLE INITIALLY DEFERRED (btree_gist 模块已安装)

Answer 1

Exclusion constraint

Additionally, table "Bar" may not contain overlapping "Timespan" values for the same "FooID" or "ParentID". I have created a trigger that fires after any INSERT, UPDATE, or DELETE that prevents overlapping ranges.

我建议你改用排除约束，这样更简单、更安全、更快:

您需要安装附加模块btree_gist第一的。请参阅此相关答案中的说明和解释:

• Store the day of the week and time?

并且您需要在表“Bar” 中冗余地包含"ParentID"，这是一个很小的代价。表定义可能如下所示:

CREATE TABLE "Foo" (
   "FooID"    serial PRIMARY KEY
   "ParentID" int4 NOT NULL REFERENCES "Parent"
   "Details1" varchar
   <b>CONSTRAINT foo_parent_foo_uni UNIQUE ("ParentID", "FooID")</b>  -- required for FK
);

CREATE TABLE "Bar" (
   "ParentID"  int4 NOT NULL,
   "FooID"     <b>int4</b> NOT NULL REFERENCES "Foo" ("FooID"),
   "Timerange" tstzrange NOT NULL,
   "Detail1"   varchar,
   "Detail2"   varchar,
   CONSTRAINT "Bar_pkey" PRIMARY KEY ("FooID", "Timerange"),
   <b>CONSTRAINT bar_foo_fk
      FOREIGN KEY ("ParentID", "FooID") REFERENCES "Foo" ("ParentID", "FooID"),
   CONSTRAINT bar_parent_timerange_excl
      EXCLUDE USING gist ("ParentID" WITH =, "Timerange" WITH &&)</b>
);

我还将 "Bar"."FooID" 的数据类型从 int8 更改为 int4。它引用 "Foo"."FooID"，这是一个 serial，即 int4。使用匹配类型 int4(或只是 integer)有几个原因，其中之一是性能。

您不再需要触发器(至少对于此任务不需要)，并且您不再创建索引 "Bar_FooID_Timerange_idx" ，因为它是由排除约束隐式创建的。

("ParentID", "FooID") 上的 btree 索引很可能有用，不过:

CREATE INDEX bar_parentid_fooid_idx ON "Bar" ("ParentID", "FooID");

相关:

• Preventing adjacent/overlapping entries with EXCLUDE in PostgreSQL

我选择 UNIQUE ("ParentID", "FooID") 而不是反过来是有原因的，因为还有另一个以 "FooID" 开头的索引任一表:

• Is a composite index also good for queries on the first field?

旁白:I never use double-quoted CaMeL-case identifiers在 Postgres 中。我这样做只是为了符合您的布局。

避免冗余列

如果您不能或不会冗余地包含 "Bar"."ParentID"，还有另一种流氓方式 - 条件是 "Foo"。 “ParentID” 从未更新。确保这一点，例如使用触发器。

你可以伪造一个IMMUTABLE函数:

CREATE OR REPLACE FUNCTION f_parent_of_foo(int)
  RETURNS int AS
'SELECT "ParentID" FROM public."Foo" WHERE "FooID" = $1'
  LANGUAGE sql IMMUTABLE;

我假设 public 对表名进行模式限定以确保。适应您的模式。

更多:

• CONSTRAINT to check values from a remotely related table (via join etc.)
• Does PostgreSQL support "accent insensitive" collations?

然后在排除约束中使用它:

   CONSTRAINT bar_parent_timerange_excl
      EXCLUDE USING gist (<b>f_parent_of_foo("FooID")</b> WITH =, "Timerange" WITH &&)

虽然节省了一个冗余的 int4 列，但约束的验证成本更高，整个解决方案取决于更多的前提条件。

处理冲突

您可以将 INSERT 和 UPDATE 包装到一个 plpgsql 函数中，并从排除约束 (23P01 exclusion_violation) 中捕获可能的异常以对其进行一些处理方式。

INSERT ...

EXCEPTION
    WHEN exclusion_violation
    THEN  -- handle conflict

完整代码示例:

• Handling EXCEPTION and return result from function

在 Postgres 9.5 中处理冲突

在 Postgres 9.5 中，您可以使用新的“UPSERT”实现直接处理 INSERT。 The documentation:

The optional ON CONFLICT clause specifies an alternative action to raising a unique violation or exclusion constraint violation error. For each individual row proposed for insertion, either the insertion proceeds, or, if an arbiter constraint or index specified by conflict_target is violated, the alternative conflict_action is taken. ON CONFLICT DO NOTHING simply avoids inserting a row as its alternative action. ON CONFLICT DO UPDATE updates the existing row that conflicts with the row proposed for insertion as its alternative action.

但是:

Note that exclusion constraints are not supported with ON CONFLICT DO UPDATE.

但是您仍然可以使用ON CONFLICT DO NOTHING，从而避免可能的exclusion_violation 异常。只需检查是否实际更新了任何行，这样成本更低:

INSERT ... 
ON CONFLICT ON CONSTRAINT bar_parent_timerange_excl DO NOTHING;

IF NOT FOUND THEN
   -- handle conflict
END IF;

此示例将检查限制为给定的排除约束。 (我在上面的表定义中为此明确命名了约束。)其他可能的异常未被捕获。