Production rule systems are conceptually simple, but there are many subtle points involved in actually using them. Some of these points and the theoretical foundations of the Postgres rule system can be found in [Stonebraker et al, ACM, 1990].
Some other database systems define active database rules. These are usually stored procedures and triggers and are implemented in Postgres as functions and triggers.
The query rewrite rule system (the "rule system" from now on) is totally different from stored procedures and triggers. It modifies queries to take rules into consideration, and then passes the modified query to the query optimizer for execution. It is very powerful, and can be used for many things such as query language procedures, views, and versions. The power of this rule system is discussed in [Ong and Goh, 1990] as well as [Stonebraker et al, ACM, 1990].
To understand how the rule system works it is necessary to know when it is invoked and what it's input and results are.
The rule system is located between the query parser and the optimizer. It takes the output of the parser, one querytree, and the rewrite rules from the pg_rewrite catalog, which are querytrees too with some extra information, and creates zero or many querytrees as result. So it's input and output are always things the parser itself could have produced and thus, anything it sees is basically representable as an SQL statement.
Now what is a querytree? It is an internal representation of an SQL statement where the single parts that built it are stored separately. These querytrees are visible when starting the Postgres backend with debuglevel 4 and typing queries into the interactive backend interface. The rule actions in the pg_rewrite system catalog are also stored as querytrees. They are not formatted like the debug output, but they contain exactly the same information.
Reading a querytree requires some experience and it was a hard time when I started to work on the rule system. I can remember that I was standing at the coffee machine and I saw the cup in a targetlist, water and coffee powder in a rangetable and all the buttons in a qualification expression. Since SQL representations of querytrees are sufficient to understand the rule system, this document will not teach how to read them. It might help to learn it and the naming conventions are required in the later following descriptions.
When reading the SQL representations of the querytrees in this document it is necessary to be able to identify the parts the statement is broken into when it is in the querytree structure. The parts of a querytree are
This is a simple value telling which command (SELECT, INSERT, UPDATE, DELETE) produced the parsetree.
The rangtable is a list of relations that are used in the query. In a SELECT statement that are the relations given after the FROM keyword.
Every rangetable entry identifies a table or view and tells by which name it is called in the other parts of the query. In the querytree the rangetable entries are referenced by index rather than by name, so here it doesn't matter if there are duplicate names as it would in an SQL statement. This can happen after the rangetables of rules have been merged in. The examples in this document will not have this situation.
This is an index into the rangetable that identifies the relation where the results of the query go.
SELECT queries normally don't have a result relation. The special case of a SELECT INTO is mostly identical to a CREATE TABLE, INSERT ... SELECT sequence and is not discussed separately here.
On INSERT, UPDATE and DELETE queries the resultrelation is the table (or view!) where the changes take effect.
The targetlist is a list of expressions that define the result of the query. In the case of a SELECT, the expressions are what builds the final output of the query. They are the expressions between the SELECT and the FROM keywords (* is just an abbreviation for all the attribute names of a relation).
DELETE queries don't need a targetlist because they don't produce any result. In fact the optimizer will add a special entry to the empty targetlist. But this is after the rule system and will be discussed later. For the rule system the targetlist is empty.
In INSERT queries the targetlist describes the new rows that should go into the resultrelation. Missing columns of the resultrelation will be added by the optimizer with a constant NULL expression. It is the expressions in the VALUES clause or the ones from the SELECT clause on INSERT ... SELECT.
On UPDATE queries, it describes the new rows that should replace the old ones. Here now the optimizer will add missing columns by inserting expressions that put the values from the old rows into the new one. And it will add the special entry like for DELETE too. It is the expressions from the SET attribute = expression part of the query.
Every entry in the targetlist contains an expression that can be a constant value, a variable pointing to an attribute of one of the relations in the rangetable, a parameter or an expression tree made of function calls, constants, variables, operators etc.
The queries qualification is an expression much like one of those contained in the targetlist entries. The result value of this expression is a boolean that tells if the operation (INSERT, UPDATE, DELETE or SELECT) for the final result row should be executed or not. It is the WHERE clause of an SQL statement.
The other parts of the querytree like the ORDER BY clause arent of interest here. The rule system substitutes entries there while applying rules, but that doesn't have much to do with the fundamentals of the rule system. GROUP BY is a special thing when it appears in a view definition and still needs to be documented.