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POIs it necessary to have an index on every combination of queryable fields in a SQL table to optimize performance?
 

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  1. POIs it necessary to have an index on every combination of queryable fields in a SQL table to optimize performance?
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    <p>If my <code>User</code> table has several fields that are queryable (say DepartmentId, GroupId, RoleId) will it make any speed difference if I create an index for each combination of those fields?</p> <p>By "queryable", I'm referring to a query screen where the end user can select records based on Department, Group or Role by selecting from a drop-down.</p> <p>At the moment, I have a index on DepartmentId, GroupId and RoleId. That's a single non-unique index per field.</p> <p>If an end user selects "anyone in Group B", the SQL looks like:</p> <pre><code>select * from User where GroupId = 2 </code></pre> <p>Having an index on GroupId should speed that up.</p> <p>But if the end user select "anyone in Group B and in Role C", the SQL would look like this:</p> <pre><code>select * from User where GroupId = 2 and RoleId = 3 </code></pre> <p>Having indexes on GroupId and RoleId individually may not make any difference, right?</p> <p>A better index for that search would be if I had one index spanning <strong>both</strong> GroupId and RoleId.</p> <p>But if that's the case, than that would mean that I would need to have an index for every combination of queryable fields. So I would need all these indexes:</p> <ul> <li>DepartmentId</li> <li>GroupId</li> <li>RoleId</li> <li>DepartmentId and GroupId</li> <li>DepartmentId and RoleId</li> <li>GroupId and RoleId</li> <li>Department Id, GroupId and RoleId</li> </ul> <p>Can anyone shed some light on this? I'm using MySQL if that makes a difference.</p>
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    1. USsohtimsso1970
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      1. POIs it necessary to have an index on every combination of queryable fields in a SQL table to optimize performance?
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    1. COPlease keep in mind that the order of the columns matter - (GroupId, RoleId) is *not* the same as (RoleId, GroupId). So, your list is missing a number of additional indicies. Otherwise, @Joe's comment about Index merging is good (DB2 also uses this, to good effect).
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      1. USClockwork-Muse
 

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