Index Search becomes faster, but additions and deletions will be slow
The index itself should occupy the space
* .Myini presence of
Introduce a tool
Explain
This is an analysis tool that can analyze a sql statement, sql statement execution efficiency can be predicted.
mysql> explain select * from emp where empno= 998776\G;
*************************** 1. row ***************************
id: 1 // representatives of the index value of the sql statement used
select_type: SIMPLE // simple, ordinary index
table: emp // now is what query tables, multi-table multi-table will be displayed
partitions: NULL
type: const // link type constant table, there are three kinds,
possible_keys: PRIMARY // might use index
key: PRIMARY // actual use of the index
key_len: 3 // index length
ref: const
rows: 1 // how much is taken out from the line in the
filtered: 100.00 // return means results need to scan the row per row (value row rows) Percentage
Extra: NULL
1 row in set, 1 warning (0.00 sec)
ERROR:
No query specified
sql statement add \ G may be longitudinally aligned
Type There are three kinds: all represents a full table scan is very slow, plus an index, so as const
Table system only one row (= system table), which is a type of link to match const
Const represents at most one match
possible_keys would choose an optimal index
Delete Index
Alter table drop PRIMARY KEY(字段)；
If the primary key, an index is automatically
This statement is an analysis will not use the index, what is the description of the way, but the analysis is not out of time.
Extra Query details
No tables, no table query
Using filesort This is the sort files, to try to avoid, is very slow.
Using temporary certain operations must use a temporary table
Common group by order by
Using where all the information in the table do not read, you can get information only through the index
According to explain information, we can see that, sql statement whether the use of the index, how much is taken out of the record, you can also see sort of way.
But if such a query:
select * from emp where ename='ouiHJB';
Or become very slow, because there is no indexed.