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When you create a PRIMARY KEY constraint, a unique clustered index on the column or columns is automatically created if a clustered index on the table does not already exist and you do not specify a unique nonclustered index. The primary key column cannot allow NULL values.
A unique index ensures that the values in the index key columns are unique. A unique constraint also guarantees that no duplicate values can be inserted into the column(s) on which the constraint is created. When a unique constraint is created a corresponding unique index is automatically created on the column(s).
We know that the unique constraint in SQL Server creates a unique SQL Server index as well. SQL Server allows us to disable an index as well without dropping it.
PostgreSQL automatically creates a unique index when a unique constraint or primary key is defined for a table. The index covers the columns that make up the primary key or unique constraint (a multicolumn index, if appropriate), and is the mechanism that enforces the constraint.
A unique key is a special case of index, acting like a regular index with added checking for uniqueness. Using SHOW INDEXES FROM customer you can see your unique keys are in fact B-tree type indexes.
A composite index on (email, user_id) is enough, you don't need a separate index on email only - MySQL can use leftmost parts of a composite index. There may be some border cases where the size of an index can slow down your queries, but you should not worry about them until you actually run into them.
As for testing index usage you should first fill your table with some data to make optimizer think it's actually worth to use that index.
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