Do you need to explicitly create an index, or is it implicit when defining the primary key? Is the answer the same for MyISAM and InnoDB?
9 Answers
The primary key is always indexed. This is the same for MyISAM and InnoDB, and is generally true for all storage engines that at all supports indices.
According to http://dev.mysql.com/doc/refman/5.0/en/constraint-primary-key.html it would appear that this is would be implicit
The primary key is implicitly indexed for both MyISAM and InnoDB. You can verify this by using EXPLAIN on a query that makes use of the primary key.
You do not have to explicitly create an index for a primary key... it is done by default.
Yes, One can think of a primary key column as any other indexed column, with the constraints of primary key brings with it.
In most of the use cases we need both primary key, and indexed column/columns in a table, because our queries to table may filter rows based on column/columns which is not primary key, in that case we usually index those column/columns as well.
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