Author: This chapter originated as part of Enhancement of the ANSI SQL Implementation of PostgreSQL, Stefan Simkovics' Master's Thesis prepared at Vienna University of Technology under the direction of O.Univ.Prof.Dr. Georg Gottlob and Univ.Ass. Mag. Katrin Seyr.
This chapter gives an overview of the internal structure of the backend of PostgreSQL. After having read the following sections you should have an idea of how a query is processed. This chapter does not aim to provide a detailed description of the internal operation of PostgreSQL, as such a document would be very extensive. Rather, this chapter is intended to help the reader understand the general sequence of operations that occur within the backend from the point at which a query is received, to the point at which the results are returned to the client.
Here we give a short overview of the stages a query has to pass in order 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.