52.1. The Path of a Query

52.1. The Path of a Query
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+ Here we give a short overview of the stages a query has to pass + to obtain a result. +

+ A connection from an application program to the PostgreSQL + server has to be established. The application program transmits a + query to the server and waits to receive the results sent back by the + server. +
+ The parser stage checks the query + transmitted by the application + program 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 query tree. It performs the + transformations given in the rule bodies. +
+ One application of the rewrite system is in the realization of + views. + 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) query tree and creates a + query plan that will be the input to the + executor. +
+ 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 path is estimated and the cheapest path is chosen. The cheapest + path is expanded into a complete plan that the executor can use. +
+ The executor recursively steps through + the plan tree and + retrieves rows 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 rows derived. +

+ In the following sections we will cover each of the above listed items + in more detail to give a better understanding of PostgreSQL's internal + control and data structures. +