SQL Query execution sequence in WHERE clause

Question

What is the excution order in Query like:

SELECT * FROM [users] WHERE [userid] = 50001 AND [username] = 'new user'

My question is what will be matched for first - [userid] or [username].

and so will affect the execution time.

Any suggesetion to improve this query will be appriciated.

Answer 1

The answer depends on the indexes that you make available to the SQL engine. If userid is indexed but username is not, the engine is likely to start with userid ; if username is indexed but userid is not, then the engine would probably start with the name; if both fields are indexed, the engine will search that index, and look up the rows by internal ids. Note that all of this is highly dependent on the RDBMS that you are using. Some engines would forego index searches altogether in favor of full table scans when the number of rows is low.

Answer 2

The database will decide what order to execute the conditions in.

Normally (but not always) it will use an index first where possible.

You can see how conditions in where or joins are needs to be optimized: - SQLStatementExecution - Example Discussion1 - Example Discussion2 - Example Discussion3

Generally speaking, the order of criteria in the WHERE clause is evaluated and optimized by the query optimizer prior to creating an execution plan. This is good. However, I would encourage you to review the query execution plan for each query prior to putting it into production.

Answer 3

SQL-based database engines will generally optimize based on the clustered (the physical order of data records) and any available indexes. MySQL and MS SQL Server (at a minimum - many others are too) are smart enough to know which order to execute filters to optimize a query.

For your purposes, it doesn't matter and the execution results will be the exact same, with the same performance, in either order.