Postgres 使用错误的索引

Question

我有一个问题：

EXPLAIN ANALYZE
SELECT CAST(DATE(associationtime) AS text) AS date ,
       cast(SUM(extract(epoch
                        FROM disassociationtime) - extract(epoch
                                                           FROM associationtime)) AS bigint) AS sessionduration,
       cast(SUM(tx) AS bigint)AS tx,
       cast(SUM(rx) AS bigint) AS rx,
       cast(SUM(dataRetries) AS bigint) AS DATA,
       cast(SUM(rtsRetries) AS bigint) AS rts,
       count(*)
FROM SESSION
WHERE ssid_id=42
  AND ap_id=1731
  AND DATE(associationtime)>=DATE('Tue Nov 04 00:00:00 MSK 2014')
  AND DATE(associationtime)<=DATE('Thu Nov 20 00:00:00 MSK 2014')
GROUP BY(DATE(associationtime))
ORDER BY DATE(associationtime);

输出是：

 GroupAggregate  (cost=0.44..17710.66 rows=1 width=32) (actual time=4.501..78.880 rows=17 loops=1)
   ->  Index Scan using session_lim_values_idx on session  (cost=0.44..17538.94 rows=6868 width=32) (actual time=0.074..73.266 rows=7869 loops=1)
         Index Cond: ((date(associationtime) >= '2014-11-04'::date) AND (date(associationtime) <= '2014-11-20'::date))
         Filter: ((ssid_id = 42) AND (ap_id = 1731))
         Rows Removed by Filter: 297425
 Total runtime: 78.932 ms

看看这一行：

Index Scan using session_lim_values_idx

如您所见，查询使用三个字段进行扫描：ssid_id、ap_id 和关联时间。我有一个索引：

ssid_pkey                  | btree | {id}
ap_pkey                    | btree | {id}
testingshit_pkey           | btree | {one,two,three}
session_date_ssid_idx      | btree | {ssid_id,date(associationtime),"date_trunc('hour'::text, associationtime)"}
session_pkey               | btree | {associationtime,disassociationtime,sessionduration,clientip,clientmac,devicename,tx,rx,protocol,snr,rssi,dataretries,rtsretries }
session_main_idx           | btree | {ssid_id,ap_id,associationtime,disassociationtime,sessionduration,clientip,clientmac,devicename,tx,rx,protocol,snr,rssi,dataretres,rtsretries}
session_date_idx           | btree | {date(associationtime),"date_trunc('hour'::text, associationtime)"}
session_date_apid_idx      | btree | {ap_id,date(associationtime),"date_trunc('hour'::text, associationtime)"}
session_date_ssid_apid_idx | btree | {ssid_id,ap_id,date(associationtime),"date_trunc('hour'::text, associationtime)"}
ap_apname_idx              | btree | {apname}
users_pkey                 | btree | {username}
user_roles_pkey            | btree | {user_role_id}
session_lim_values_idx     | btree | {date(associationtime)}

它叫做session_date_ssid_apid_idx。但是为什么查询使用错误的索引？

session_date_ssid_apid_idx：

------------+-----------------------------+-------------------------------------------
 ssid_id    | integer                     | ssid_id
 ap_id      | integer                     | ap_id
 date       | date                        | date(associationtime)
 date_trunc | timestamp without time zone | date_trunc('hour'::text, associationtime)

session_lim_values_idx：

date    | date | date(associationtime)

你会创建什么索引？

UPD： \d session

 --------------------+-----------------------------+------------------------------------------------------
 id                 | integer                     | NOT NULL DEFAULT nextval('session_id_seq'::regclass)
 ssid_id            | integer                     | NOT NULL
 ap_id              | integer                     | NOT NULL
 associationtime    | timestamp without time zone | NOT NULL
 disassociationtime | timestamp without time zone | NOT NULL
 sessionduration    | character varying(100)      | NOT NULL
 clientip           | character varying(100)      | NOT NULL
 clientmac          | character varying(100)      | NOT NULL
 devicename         | character varying(100)      | NOT NULL
 tx                 | integer                     | NOT NULL
 rx                 | integer                     | NOT NULL
 protocol           | character varying(100)      | NOT NULL
 snr                | integer                     | NOT NULL
 rssi               | integer                     | NOT NULL
 dataretries        | integer                     | NOT NULL
 rtsretries         | integer                     | NOT NULL
╚эфхъё√:
    "session_pkey" PRIMARY KEY, btree (associationtime, disassociationtime, sessionduration, clientip, clientmac, devicename, tx, rx, protocol, snr, rssi, dataretries, rtsretries)
    "session_date_ap_ssid_idx" btree (ssid_id, ap_id, associationtime)
    "session_date_apid_idx" btree (ap_id, date(associationtime), date_trunc('hour'::text, associationtime))
    "session_date_idx" btree (date(associationtime), date_trunc('hour'::text, associationtime))
    "session_date_ssid_apid_idx" btree (ssid_id, ap_id, associationtime)
    "session_date_ssid_idx" btree (ssid_id, date(associationtime), date_trunc('hour'::text, associationtime))
    "session_lim_values_idx" btree (date(associationtime))
    "session_main_idx" btree (ssid_id, ap_id, associationtime, disassociationtime, sessionduration, clientip, clientmac, devicename, tx, rx, protocol, snr, rssi, dataretries, rtsretries)

Answer 1

ssid_id 和 ap_id 的谓词中非常常见的值可以使 Postgres 更便宜地选择较小的索引 session_lim_values_idx（只有 1 个 date 列）而不是看似更合适但更大的索引 @987654329 @（4 列）并过滤其余部分。

在您的情况下，大约 4% 的行具有 ssid_id=42 AND ap_id=1731。这通常不应该保证切换到较小的索引。但还有其他几个可能会影响规模的因素，基本上是成本设置和统计数据。详情：

• Keep PostgreSQL from sometimes choosing a bad query plan

怎么办？

• 如果您尚未按照linked the answer above 中的建议调整您的费用设置。

• 增加所涉及列ssid_id、ap_id 的统计目标并运行ANALYZE：

  • Check statistics targets in PostgreSQL

  这里有一个特殊因素：Postgres 收集索引中表达式的单独统计数据。检查：

  SELECT * FROM pg_statistic
  WHERE starelid = 'session_date_ssid_apid_idx'::regclass;
  

  您会找到表达式date(associationtime) 的专用行。更多详情：

  • Index that is not used, yet influences query

• 通过删除第 4 列 "date_trunc('hour'::text, associationtime)，使索引 session_date_ssid_apid_idx 更具吸引力（更小）。查看您后来添加的表定义，您已经这样做了。

• 我宁愿使用转换的标准语法：cast(associationtime AS date)，而不是函数语法date(associationtime)。一点都不重要，我只是知道正常工作的标准方法。您可以在查询中使用简写语法associationtime::date，它与表达式索引兼容，但在索引定义中使用详细形式。

另外，使用EXPLAIN ANALYZE 测试哪个查询计划实际上更快，方法是仅删除/重新创建要测试的索引。然后你会看到 Postgres 是否选择了最好的计划。

你有很多索引，我会检查它们是否都被实际使用并删除其余的。索引有维护成本，如果可能的话，专注于更少的索引通常是有益的（更容易放入缓存中，并且可能在需要时已经缓存）。权衡成本与收益。

一边

我会使用：

SUM(extract(epoch FROM disassociationtime
                     - associationtime)::int) AS sessionduration