It is over head, but it caches the execution plan for multiple runs of the
script. So different users with different data will use the same cached
query on the database. Saving processing time. It also prevents SQL
injection on the fly because you are indicating what data type each place
holder will need to accept.
No, it's per session.
http://dev.mysql.com/doc/refman/5.0/en/sql-syntax-prepared-statements.html
The scope of a prepared statement is the client session within which it is created. Other clients cannot see it.
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