How many columns are in a composite index in MySQL?
MySQL allows you to create a composite index that consists of up to 16 columns. A composite index is also known as a multiple-column index. The query optimizer uses the composite indexes for queries that test all columns in the index, or queries that test the first columns, the first two columns, and so on.
How does range condition extraction work in MySQL?
MySQL performs an additional check to filter out rows that satisfy the range condition but not the full WHERE clause. The range condition extraction algorithm can handle nested AND / OR constructs of arbitrary depth, and its output does not depend on the order in which conditions appear in WHERE clause.
What happens in the extraction process in MySQL?
During the extraction process, conditions that cannot be used for constructing the range condition are dropped, conditions that produce overlapping ranges are combined, and conditions that produce empty ranges are removed. Consider the following statement, where key1 is an indexed column and nonkey is not indexed:
How are composite indexes used in query optimizer?
The query optimizer uses the composite indexes for queries that test all columns in the index, or queries that test the first columns, the first two columns, and so on. If you specify the columns in the right order in the index definition, a single composite index can speed up these kinds of queries on the same table.
When do you use an index in MySQL?
If an index exists on (col1, col2, col3) , only the first two queries use the index. The third and fourth queries do involve indexed columns, but do not use an index to perform lookups because (col2) and (col2, col3) are not leftmost prefixes of (col1, col2, col3) .
Can a multiple column index be considered a sorted array?
A multiple-column index can be considered a sorted array, the rows of which contain values that are created by concatenating the values of the indexed columns. As an alternative to a composite index, you can introduce a column that is “hashed” based on information from other columns.
Do you have to order Index ( x, y ) in MySQL?
That is, you must order it INDEX (x,y). For WHERE x = 1 AND y = 2 (both = ), it does not matter whether you have INDEX (x,y) or INDEX (y,x) . Another tidbit: With ENGINE=InnoDB, the PRIMARY KEY column (s) are implicitly tacked onto each secondary key.
When to use id, column or index in MySQL?
So, if id is highly selective, and column has low selectivity, you’d want the index to be (id, column), however if column has high selectivity, and id has low selectivity, you’d probably benefit from having the index defined as (column, id).
When to use the name index in MySQL?
First, the name index can be used for lookups in the queries that specify a lastName value because the lastName column is the leftmost prefix of the index. Second, the name index can be used for queries that specify values for the combination of the lastName and firstName values.