分析 PostgreSQL 执行计划

Posted 2023-04-14

【中文标题】分析 PostgreSQL 执行计划

【英文标题】：Analysing the PostgreSQL execution Plan

【发布时间】：2019-10-15 13:02:15

【问题描述】：

我试图了解 PostgreSQL 如何使用索引以及它们的执行计划如何。

我执行了以下查询，

EXPLAIN(VERBOSE,ANALYZE)
SELECT SUM("Orders"."order_subtotal_net_after_discount") AS "Total sum of Order Subtotal Net After Discount"
FROM "ol"."orders_test" AS "Orders"
LEFT OUTER JOIN "ol"."ga_transactions" AS "Ga Transactions" ON "Ga Transactions"."transaction_id" = "Orders"."order_number"
WHERE ( to_char(created_at_order,'YYYY-MM-DD')=to_char(now(),'YYYY-MM-DD')
       AND "Orders"."order_state_2"<>'canceled'
      AND "Orders"."order_state_2" <> 'pending_payment'
      AND "Orders"."order_state_1" <> 'canceled'
      AND "Orders"."order_state_1" <> 'pending_payment'    
      AND "Orders"."order_state_1" <> 'closed'
      AND "Orders"."order_state_2" <> 'closed'
       )

得到了下面的执行计划，我发现下面的过滤器发生了顺序扫描

Filter: ((("Orders".order_state_2)::text <> 'canceled'::text) AND (("Orders".order_state_2)::text <> 'pending_payment'::text) AND (("Orders".order_state_1)::text <> 'canceled'::text) AND (("Orders".order_state_1)::text <> 'pending_payment'::text) AND (("Orders".order_state_1)::text <> 'closed'::text) AND (("Orders".order_state_2)::text <> 'closed'::text) AND (to_char("Orders".created_at_order, 'YYYY-MM-DD'::text) = to_char(now(), 'YYYY-MM-DD'::text)))"

整个查询计划

Aggregate  (cost=157385.36..157385.37 rows=1 width=32) (actual time=840.221..840.221 rows=1 loops=1)
  Output: sum("Orders".order_subtotal_net_after_discount)
  ->  Nested Loop Left Join  (cost=0.42..157378.56 rows=2717 width=6) (actual time=0.017..840.095 rows=470 loops=1)
        Output: "Orders".order_subtotal_net_after_discount
        ->  Seq Scan on ol.orders_test "Orders"  (cost=0.00..148623.91 rows=2717 width=16) (actual time=0.009..838.144 rows=470 loops=1)
              Output: "Orders".rno, "Orders".order_id, "Orders".order_number, "Orders".invoice_id, "Orders".invoice_number, "Orders".store_id, "Orders".customer_id, "Orders".real_customer_id, "Orders".shipping_address_id, "Orders".order_state_1, "Orders".order_state_2, "Orders".invoice_state, "Orders".shipping_street, "Orders".shipping_city, "Orders".shipping_postcode, "Orders".shipping_country_id, "Orders".shipping_description, "Orders".coupon_code, "Orders".first_order_of_customer, "Orders".payment_method, "Orders".order_subtotal_net, "Orders".order_subtotal_net_after_discount, "Orders".order_subtotal, "Orders".order_shipment, "Orders".order_shipping_tax, "Orders".order_discount, "Orders".order_tax_total, "Orders".order_grand_total, "Orders".order_total_paid, "Orders".order_refunded_total, "Orders".order_total_open, "Orders".invoice_subtotal_net, "Orders".invoice_subtotal_net_after_discount, "Orders".invoice_subtotal, "Orders".invoice_shipment, "Orders".invoice_shipping_tax, "Orders".invoice_discount, "Orders".invoice_tax_total, "Orders".invoice_grand_total, "Orders".invoice_refunded_total, "Orders".created_at_order, "Orders".created_at_invoice, "Orders".updated_at_invoice, "Orders".customer_email, "Orders".row_number, "Orders".nthorder, "Orders".time_since_last_order, "Orders".time_since_first_order
              Filter: ((("Orders".order_state_2)::text <> 'canceled'::text) AND (("Orders".order_state_2)::text <> 'pending_payment'::text) AND (("Orders".order_state_1)::text <> 'canceled'::text) AND (("Orders".order_state_1)::text <> 'pending_payment'::text) AND (("Orders".order_state_1)::text <> 'closed'::text) AND (("Orders".order_state_2)::text <> 'closed'::text) AND (to_char("Orders".created_at_order, 'YYYY-MM-DD'::text) = to_char(now(), 'YYYY-MM-DD'::text)))
              Rows Removed by Filter: 654356
        ->  Index Only Scan using ga_transactions_transaction_id_idx on ol.ga_transactions "Ga Transactions"  (cost=0.42..3.21 rows=1 width=10) (actual time=0.004..0.004 rows=0 loops=470)
              Output: "Ga Transactions".transaction_id
              Index Cond: ("Ga Transactions".transaction_id = ("Orders".order_number)::text)
              Heap Fetches: 0
Planning Time: 0.540 ms
Execution Time: 840.255 ms

我创建了下面的索引，但查询似乎没有使用这个索引，

是否必须在 INCLUDE 列表中包含所有输出列？

如果我添加更多索引，它会增加更新时间。那么有没有最好的方法来识别一个表的有效索引？

CREATE INDEX "IX_states"
    ON ol.orders_test USING btree
    (order_state_2   ASC NULLS LAST, order_state_1   ASC NULLS LAST, created_at_order   ASC NULLS LAST)
    INCLUDE(rno, order_id, order_number, invoice_id, invoice_number, store_id, customer_id, real_customer_id, shipping_address_id, invoice_state, shipping_street, shipping_city, shipping_postcode, shipping_country_id, shipping_description, coupon_code, first_order_of_customer, payment_method, order_subtotal_net_after_discount, created_at_invoice, customer_email, nthorder, time_since_last_order, time_since_first_order)
    WITH (FILLFACTOR=90)
    TABLESPACE pg_default;

【问题讨论】：

你总是使用那些 `oder_state ...`` 条件吗？

@a_horse_with_no_name 是的，在大多数查询中都会出现这种情况。

created_at_order是什么数据类型？

数据类型为“timestamp without time zone”，但大多使用 TO_CHAR() 函数转换得到所需的日期格式。

【参考方案1】：

没有必要在索引中包含所有列。索引用于快速查找几行。让一切突然变得更快的并不是灵丹妙药。如果您在索引中包含所有表列，则扫描索引将花费与执行 Seq Scan 一样多的时间

你肯定想索引created_at_order。

create index on orders_test (created_at_order);

但是，您的表达式 to_char(created_at_order,'YYYY-MM-DD') 不会使用该索引。最好的办法是改变你的状况：

where created_at_order >= current_date and created_at_order < current_date + 1

这将利用上述索引。如果您不想使用这样的表达式，那么您需要一个专门的表达式索引。我不会将其转换为文本值，而是索引日期值：

create index on orders_test ( (created_at_order::date) );

索引日期值还具有索引更小的优点。 date 只占用 4 个字节，而字符串 '2019-10-15' 占用 10 个字节的存储空间。

那么以下将使用该索引：

where created_at_order::date = current_date

如果对状态列的附加限制进一步减少了行数，您可以为此使用过滤索引：

create index on orders_test ( (created_at_order::date) )
WHERE order_state_2 <>'canceled'
  AND order_state_2  <> 'pending_payment'
  AND order_state_1  <> 'canceled'
  AND order_state_1  <> 'pending_payment'    
  AND order_state_1  <> 'closed'
  AND order_state_2  <> 'closed';

这将使索引更小并更快地在索引中查找行。但是，该索引将不再用于不包含这些条件的查询。

如果您不是一直使用所有这些条件，请将它们减少到您一直使用的那些条件。

如果这些附加条件并没有真正减少行数，因为“今天创建”的条件已经非常严格，那么您可以将它们排除在外。

【讨论】：

非常感谢您的简短解释，我将按照您的说明更改查询并创建索引。那么 INCLUDE 列，不必添加任何内容作为 INCLUDED 列吗？

INCLUDE 并不是必须的。您可能希望在两列上创建索引，例如(created_at_order, order_number) 或 (created_at_order::date, order_number) 因为 order_number 用于连接。

【参考方案2】：

您使用created_at_order 作为时间戳创建了索引，但在WHERE 中您正在使用函数转换和比较字符串。 我建议您将WHERE clausule 编辑为如下内容：

WHERE created_at_order BETWEEN now() AND TIMESTAMP 'today'