CREATE INDEX

toms1981

I wish to know how I going to know am I create a correct index? Can any provide some examples for creating a proper index? THANKS...

huyuhui001

If you define a PK of a table,the DB will create unique index of this table using PK when you create it. This index is managed by system and you can not drop it.
You can also create yourself indexes ,but more indexes of a table will effect your write performance

toms1981

ya I know but currently my query was slow that why I need to create index in order to make retrive data faster. Actually, may I know the reason why the more indexes we created it will affect the performance when we insert data into a table?

huyuhui001

If a table has many indexes,it will be slower when you insert records into the tables. If you want to improve your load or import performance, you should chose right indexes.
eg:
table A,
PK: col1,col2,col3
index1(col1,col2,col3)
index2(col1,col2)
index2(col1,col3)
When you execute the command "select ... from A where col1=x and col2=x", the index1 is used.