Shulou(Shulou.com)05/31 Report--

This article is to share with you what are the basic knowledge points of InnoDB. The editor thinks it is very practical, so share it with you as a reference and follow the editor to have a look.

1. About count (*)

Knowledge: MyISAM will directly store the total number of lines, InnoDB will not, need to scan by line.

The subtext is that for select count (*) from t; if the amount of data is large, MyISAM will return instantly, while InnoDB will scan line by line.

Practice: for tables with a large amount of data, InnoDB should not easily select count (*), which consumes a lot of performance.

Common pit: only when querying the total number of rows in the whole table, MyISAM will directly return the result. When the where condition is added, the two storage engines handle it in a similar way.

For example:

T_user (uid, uname, age, sex)

Uid competition

Age index

Select count (*) where age

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