A connection from an application program to the PostgreSQL
server has to be established. The application program transmits a
query to the server and receives the results sent back by the server.
The parser stage checks the query
transmitted by the application
program (client) for correct syntax and creates
a query tree.
The rewrite system takes
the query tree created by the parser stage and looks for
any rules (stored in the
system catalogs) to apply to
the querytree and performs the
transformations given in the rule bodies.
One application of the rewrite system is given in the realization of
Whenever a query against a view
(i.e. a virtual table) is made,
the rewrite system rewrites the user's query to
a query that accesses the base tables given in
the view definition instead.
The planner/optimizer takes
the (rewritten) querytree and creates a
queryplan that will be the input to the
It does so by first creating all possible paths
leading to the same result. For example if there is an index on a
relation to be scanned, there are two paths for the
scan. One possibility is a simple sequential scan and the other
possibility is to use the index. Next the cost for the execution of
each plan is estimated and the
cheapest plan is chosen and handed back.
The executor recursively steps through
the plan tree and
retrieves tuples in the way represented by the plan.
The executor makes use of the
storage system while scanning
relations, performs sorts and joins,
evaluates qualifications and finally hands back the tuples derived.