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What happens to the Order of the columns in an index?
Altering the index in order of selectivity doesn’t affect either the expected number of key comparisons from the binary search or the number of pages that need to be navigated to do an index seek. At best it might marginally speed up the key comparison itself.
Can you use an index in MySQL order by column?
However, MySQL cannot use an index for mixed ASC,DESC order by ( SELECT * FROM foo ORDER BY bar ASC, pants DESC ). Sharing your query and CREATE TABLE statement would help us answer your question more specifically. For hints on how to optimize ORDER BY clauses: http://dev.mysql.com/doc/refman/5.1/en/order-by-optimization.html
When to use partitioned aligned indexes in SQL?
If you don’t specify the filegroup name (ie [PRIMARY]) then by default SQL will assume you wanted a partitioned NC index and automatically align it to the same partition function as that of the underlying table . (I hope that’s what you wanted says SQL? Too bad if it takes 6 hours to create it before you notice!)
When do you put the partitioning column in the index?
When partitioning a unique nonclustered index, the index key must contain the partitioning column. When partitioning a nonunique, nonclustered index, SQL Server adds the partitioning column by default as a nonkey (included) column of the index to make sure the index is aligned with the base table.
When to use an index on the mostselective column?
With that knowledge in hand you can look at the index definitions again. An index on (MostSelective, SecondMost, Least) will be effective only when MostSelective column is specified. But that being the most selective, the relevance of the inner columns will quickly degrade.
When to choose columns in a clustered index?
What I want to say is how to choose columns when : Creating clustered index. Creating non-clustered index. Creating non-clustered with included column (s). The above example is just to illustrate the idea of the question.
Can a 3rd query use both columns of index?
In particular, your 3rd query can use both columns of INDEX (CinemaID, Showtime), but not both columns of INDEX (Showtime, CinemaID). It is easy to see this: Think of writing out two lists of cinemas and showtimes.