Performance 了解复杂SQL联接的性能_Performance_Sqlite_Join_Subquery_Big O

Performance 了解复杂SQL联接的性能

performance sqlite join big-o

Performance 了解复杂SQL联接的性能,performance,sqlite,join,subquery,big-o,Performance,Sqlite,Join,Subquery,Big O,我试图通过重新构造查询来理解我在SQLite数据库上解决的性能问题查询的任务是筛选协议最新修订的列表。此外，某些协议可能不会发布，这将从搜索中排除。表的建模如下所示： -- Describe PROTOCOL CREATE TABLE protocol ( id CHAR(36) NOT NULL, revision CHAR(36) NOT NULL, timestamp DATETIME, name TEXT, PRIMARY KEY (revision) )

我试图通过重新构造查询来理解我在SQLite数据库上解决的性能问题

查询的任务是筛选协议最新修订的列表。此外，某些协议可能不会发布，这将从搜索中排除。表的建模如下所示：

-- Describe PROTOCOL
CREATE TABLE protocol (
  id CHAR(36) NOT NULL, 
  revision CHAR(36) NOT NULL, 
  timestamp DATETIME,
  name TEXT, 
  PRIMARY KEY (revision)
)

-- Describe PROTOCOLSTATUS
CREATE TABLE protocolstatus (
  uuid CHAR(36) NOT NULL, 
  status TEXT, 
  timestamp DATETIME, 
  protocol_id CHAR(36) NOT NULL, 
  PRIMARY KEY (uuid), 
  FOREIGN KEY(protocol_id) REFERENCES protocol (revision)
)

protocolstatus

中的条目数可以预期为

协议

中条目数的常数倍（

#p

），即使随着时间的推移，该常数可能会变得更大

在一个有300个协议和900个状态的数据库中，执行较慢的查询大约需要4秒钟。我还注意到，如果我不使用distinct，结果会重复很多次

使用子查询重新构造查询将整个过程加快到可行的响应时间（显示这些查询的输出）。

-- slow query: O((#p)^4) ?, Run Time: real 4.335 user 4.331110 sys 0.005220

SELECT DISTINCT
  protocol.id AS protocol_id,
  protocol.revision AS protocol_revision,
  protocol.timestamp AS protocol_timestamp
FROM
  protocolstatus, -- O(#p) |
  protocol -- O(#p)        | => O(#p^2) entries

JOIN (
  SELECT
    protocol.id AS id, max(protocol.timestamp) AS ts_max 
  FROM protocol
  GROUP BY protocol.id
) AS anon_1 -- O(#p)
ON protocol.timestamp = anon_1.ts_max

JOIN (
  SELECT
    max(protocolstatus.timestamp) AS max_1,
    protocolstatus.protocol_id AS protocol_id 
  FROM protocolstatus
  GROUP BY protocolstatus.protocol_id
) AS anon_2 -- O(#p)
ON protocol.revision = anon_2.protocol_id 

WHERE
  protocolstatus.status = 'approved'
  AND lower(protocol.name) LIKE lower('%c%')

ORDER BY protocol.name;

-- fast query: O(#p^3)? Run Time: real 0.042 user 0.041540 sys 0.000767
SELECT
  protocol.id AS protocol_id,
  protocol.revision AS protocol_revision,
  protocol.timestamp AS protocol_timestamp
FROM protocol -- O(#p)

JOIN (
  SELECT
    protocol.id AS id,
    max(protocol.timestamp) AS ts_max 
  FROM protocol
  GROUP BY protocol.id
) AS anon_1  -- query: O(#p), results: O(#p)
ON protocol.timestamp = anon_1.ts_max 

JOIN (
  SELECT
    protocolstatus.protocol_id AS protocol_id,
    protocolstatus.status AS status,
    protocolstatus.timestamp AS timestamp 
  FROM protocolstatus -- O(#p)

  JOIN (
    SELECT
      max(protocolstatus.timestamp) AS timestamp,
      protocolstatus.protocol_id AS protocol_id 
    FROM protocolstatus
    GROUP BY protocolstatus.protocol_id
  ) AS anon_3 -- O(#p)
  ON protocolstatus.timestamp = anon_3.timestamp 

  WHERE protocolstatus.status = "approved"
) AS anon_2 -- query: O(#p^2), results: O(#p)
ON protocol.revision = anon_2.protocol_id 

WHERE lower(protocol.name) LIKE lower("%c%") ORDER BY protocol.name;

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