mysql查詢INFORMATION_SCHEMA表很慢的性能優化

最近發現，我們有些環境的tomcat應用啟動非常緩慢，大部分在3-5分鍾，有個測試環境更加階段，要十幾分鍾才能啟動完成。經過仔細分析，是一個查詢INFORMATION_SCHEMA庫中數據字典信息的查詢異常緩慢，該語句如下：

SELECT
    c.COLUMN_NAME,
    c.TABLE_NAME
FROM
    information_schema.TABLE_CONSTRAINTS AS t,
    information_schema.KEY_COLUMN_USAGE AS c
WHERE
    t.TABLE_NAME = c.TABLE_NAME
AND t.TABLE_SCHEMA = c.CONSTRAINT_SCHEMA
AND t.CONSTRAINT_SCHEMA = 'hs_tatrade2'
AND t.CONSTRAINT_TYPE = 'PRIMARY KEY'

以前從來都沒遇到這種問題，也很少關心mysql數據字典查詢的性能問題，因為幾乎沒有遇到過。

查看show processlist，一直在after opening table等待。。。。

看了下執行計划以及information_schema中表結構的定義，因為都是內存表，都沒有索引，這兩張表都只有數百條記錄，按說即使沒有索引也不會這么慢。。。

經網上搜尋，有人有不少帖子提及是因為innodb_stats_on_metadata=ON導致查詢information_schema時更新統計信息的原因。經測試，不是這個原因（其實，我現在相信網上80%以上的所謂分析帖子都是理論上的測試，並不是真正遇到，尤其是所謂的很多專家）。

再次尋找到mysql官方文檔，https://dev.mysql.com/doc/refman/5.7/en/information-schema-optimization.html，如下：

8.2.3 Optimizing INFORMATION_SCHEMA Queries

 

1) Try to use constant lookup values for database and table names in the WHERE clause

You can take advantage of this principle as follows:

• To look up databases or tables, use expressions that evaluate to a constant, such as literal values, functions that return a constant, or scalar subqueries.

• Avoid queries that use a nonconstant database name lookup value (or no lookup value) because they require a scan of the data directory to find matching database directory names.

• Within a database, avoid queries that use a nonconstant table name lookup value (or no lookup value) because they require a scan of the database directory to find matching table files.

This principle applies to the INFORMATION_SCHEMA tables shown in the following table, which shows the columns for which a constant lookup value enables the server to avoid a directory scan. For example, if you are selecting from TABLES, using a constant lookup value for TABLE_SCHEMA in the WHERE clause enables a data directory scan to be avoided.

Table	Column to specify to avoid data directory scan	Column to specify to avoid database directory scan
COLUMNS	TABLE_SCHEMA	TABLE_NAME
KEY_COLUMN_USAGE	TABLE_SCHEMA	TABLE_NAME
PARTITIONS	TABLE_SCHEMA	TABLE_NAME
REFERENTIAL_CONSTRAINTS	CONSTRAINT_SCHEMA	TABLE_NAME
STATISTICS	TABLE_SCHEMA	TABLE_NAME
TABLES	TABLE_SCHEMA	TABLE_NAME
TABLE_CONSTRAINTS	TABLE_SCHEMA	TABLE_NAME
TRIGGERS	EVENT_OBJECT_SCHEMA	EVENT_OBJECT_TABLE
VIEWS	TABLE_SCHEMA	TABLE_NAME

意思就是查詢上述這些表的時候，務必帶上TABLE_SCHEMA=或者XXX_SCHEMA，以避免數據目錄掃描。而我們的場景剛好是TABLE_CONSTRAINTS沒有使用TABLE_SCHEMA，雖然關聯使用了TABLE_SCHEMA，但這是沒有用的。

經過將SQL更改為如下：

SELECT
    c.COLUMN_NAME,
    c.TABLE_NAME
FROM
    information_schema.TABLE_CONSTRAINTS AS t,
    information_schema.KEY_COLUMN_USAGE AS c
WHERE
    t.TABLE_NAME = c.TABLE_NAME
AND t.TABLE_SCHEMA = c.CONSTRAINT_SCHEMA
AND t.TABLE_SCHEMA = 'hs_tatrade2'
AND c.TABLE_SCHEMA = 'hs_tatrade2'
AND t.CONSTRAINT_TYPE = 'PRIMARY KEY'

瞬間就飛快了。