How to use index in select statement

Question

Lets say in the employee table  I have created an index idx name  on the emp name column of the table   Do I need to explicitly specify the index name in select clause or it will automatically used to speed up queries   If it is required to be specified in the select clause  What is the syntax for using index in select query

User · Answer

Generally  when you create an index on a table  database will automatically use that index while searching for data in that table  You don t need to do anything about that   However  in MSSQL  you can specify an index hint which can specify that a particular index should be used to execute this query  More information about this can be found here   Index hint is also seems to be available for MySQL  Thanks to Tudor Constantine

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In general  the index will be used if the assumed cost of using the index  and then possibly having to perform further bookmark lookups is lower than the cost of just scanning the entire table   If your query is of the form   SELECT Name from Table where Name    Boris    And 1 row out of 1000 has the name Boris  it will almost certainly be used  If everyone s name is Boris  it will probably resort to a table scan  since the index is unlikely to be a more efficient strategy to access the data   If it s a wide table  lot s of columns  and you do   SELECT   from Table where Name    Boris    Then it may still choose to perform the table scan  if it s a reasonable assumption that it s going to take more time retrieving the other columns from the table than it will to just look up the name  or again  if it s likely to be retrieving a lot of rows anyway

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The optimiser will judge if the use of your index will make your query run faster  and if it is  it will use the index    Depending on your RDBMS you can force the use of an index  although it is not recommended unless you know what you are doing    In general you should index columns that you use in table join s and where statements

User · Answer

The index hint is only available for Microsoft Dynamics database servers  For traditional SQL Server  the filters you define in your  Where  clause should persuade the engine to use any relevant indices    Provided the engine s execution plan can efficiently identify how to read the information  whether a full table scan or an indexed scan  - it must compare the two before executing the statement proper  as part of its built-in performance optimiser   However  you can force the optimiser to scan by using something like      Select        From     yourtable  With  Index 0       Where         Or to seek a particular index by using something like      Select        From     yourtable  With  Index 1       Where         The choice is yours  Look at the table s index properties in the object panel to get an idea of which index you want to use  It ought to match your filter s    For best results  list the filters which would return the fewest results first  I don t know if I m right in saying  but it seems like the query filters are sequential  if you get your sequence right  the optimiser shouldn t have to do it for you by comparing all the combinations  or at least not begin the comparison with the more expensive queries

User · Answer

Good question    Usually the DB engine should automatically select the index to use based on query execution plans it builds  However  there are some pretty rare cases when you want to force the DB to use a specific index   To be able to answer your specific question you have to specify the DB you are using   For MySQL  you want to read the Index Hint Syntax documentation on how to do this

User · Answer

By using the column that the index is applied to within your conditions  it will be included automatically  You do not have to use it  but it will speed up queries when it is used   SELECT   FROM TABLE WHERE attribute    value    Will use the appropriate index

User · Answer

How to use index in select statement  this way       SELECT   FROM table1 USE INDEX  col1 index col2 index      WHERE col1 1 AND col2 2 AND col3 3      SELECT   FROM table1 IGNORE INDEX  col3 index  WHERE col1 1 AND col2 2 AND col3 3      SELECT   FROM t1 USE INDEX  i1  IGNORE INDEX  i2  USE INDEX  i2     And many more ways check this  Do I need to explicitly specify    No  no Need to specify explicitly  DB engine should automatically select the index to use based on query execution plans it builds from  Tudor Constantin answer  The optimiser will judge if the use of your index will make your query run faster  and if it is  it will use the index  from  niktrl answer

User · Answer

If you want to test the index to see if it works  here is the syntax   SELECT   FROM Table WITH INDEX Index Name     The WITH statement will force the index to be used

[sql] How to use index in select statement?

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