Select count Efficiency of Mysql Optimization

 

1. select count (*) FROM tablename is the optimal choice under any circumstances;

2. Minimize select count (*) FROM tablename where col = 'value' queries;

3. prevent the appearance of select count (COL) FROM tablename.

 

 

COUNT (*) and COUNT (COL)

I searched the internet and found that there are various statements:

For example, COUNT (COL) is faster than COUNT;

COUNT (*) is faster than COUNT (COL;

Another funny friend said that this is actually a matter of character.

Without the WHERE restriction, the values of COUNT (*) and COUNT (COL) are equivalent;

However, with the WHERE condition, COUNT (*) is much faster than COUNT (COL;

The specific data is as follows:

Mysql & gt; select count (*) FROM cdb_posts where fid = 604;

+ ---- +

| COUNT (fid) |

+ ---- +

| 1, 79000 |

+ ---- +

1 row in set (0.03 sec)

Mysql> select count (tid) FROM cdb_posts where fid = 604;

+ ---- +

| COUNT (tid) |

+ ---- +

| 1, 79000 |

+ ---- +

1 row in set (0.33 sec)

Mysql & gt; select count (pid) FROM cdb_posts where fid = 604;

+ ---- +

| COUNT (pid) |

+ ---- +

| 1, 79000 |

+ ---- +

1 row in set (0.33 sec)

COUNT (*) is usually used to scan the primary key index, but COUNT (COL) is not necessarily the same. In addition, the former is the total number of all records in the statistical table, the latter is the number of records that match all the COL in the calculation table. There are also differences.

WHERE in COUNT

To put it simply, if there is no WHERE limit in COUNT, MySQL will directly return the total number of rows saved.

In the case of WHERE restrictions, you always need to traverse the full table of MySQL.

Optimization summary:

1. select count (*) FROM tablename is the optimal choice under any circumstances;

2. Minimize select count (*) FROM tablename where col = 'value' queries;

3. prevent the appearance of select count (COL) FROM tablename.