| | Chapter 12. The Rule System This chapter discusses the rule system in
EnterpriseDB. Production rule systems
are conceptually simple, but there are many subtle points
involved in actually using them. Some other database systems define active database rules, which
are usually stored procedures and triggers. In
EnterpriseDB, these can be implemented
using functions and triggers as well. The rule system (more precisely speaking, the query rewrite rule
system) 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 planner for planning and
execution. It is very powerful, and can be used for many things
such as query language procedures, views, and versions. To understand how the rule system works it is necessary to know
when it is invoked and what its input and results are. The rule system is located between the parser and the planner.
It takes the output of the parser, one query tree, and the user-defined
rewrite rules, which are also
query trees with some extra information, and creates zero or more
query trees as result. So its 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 query tree? It is an internal representation of an
SQL statement where the single parts that it is
built from are stored separately. These query trees can be shown
in the server log if you set the configuration parameters
debug_print_parse,
debug_print_rewritten, or
debug_print_plan. The rule actions are also
stored as query trees, in the system catalog
pg_rewrite. They are not formatted like
the log output, but they contain exactly the same information. Reading a raw query tree requires some experience. But since
SQL representations of query trees are
sufficient to understand the rule system, this chapter will not
teach how to read them. When reading the SQL representations of the
query trees in this chapter it is necessary to be able to identify
the parts the statement is broken into when it is in the query tree
structure. The parts of a query tree are
- the command type
This is a simple value telling which command
(SELECT, INSERT,
UPDATE, DELETE) produced
the query tree.
- the range table
The range table is a list of relations that are used in the query.
In a SELECT statement these are the relations given after
the FROM key word.
Every range table entry identifies a table or view and tells
by which name it is called in the other parts of the query.
In the query tree, the range table entries are referenced by
number 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 range tables of rules
have been merged in. The examples in this chapter will not have
this situation.
- the result relation
This is an index into the range table 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 followed by a
INSERT ... SELECT and is not discussed
separately here.
For INSERT, UPDATE, and
DELETE commands, the result relation is the table
(or view!) where the changes are to take effect.
- the target list
The target list is a list of expressions that define the
result of the query. In the case of a
SELECT, these expressions are the ones that
build the final output of the query. They correspond to the
expressions between the key words SELECT
and FROM. (* is just an
abbreviation for all the column names of a relation. It is
expanded by the parser into the individual columns, so the
rule system never sees it.)
DELETE commands don't need a target list
because they don't produce any result. In fact, the planner will
add a special CTID entry to the empty target list, but
this is after the rule system and will be discussed later; for the
rule system, the target list is empty.
For INSERT commands, the target list describes
the new rows that should go into the result relation. It consists of the
expressions in the VALUES clause or the ones from the
SELECT clause in INSERT
... SELECT. The first step of the rewrite process adds
target list entries for any columns that were not assigned to by
the original command but have defaults. Any remaining columns (with
neither a given value nor a default) will be filled in by the
planner with a constant null expression.
For UPDATE commands, the target list
describes the new rows that should replace the old ones. In the
rule system, it contains just the expressions from the SET
column = expression part of the command. The planner will handle
missing columns by inserting expressions that copy the values from
the old row into the new one. And it will add the special
CTID entry just as for DELETE, too.
Every entry in the target list contains an expression that can
be a constant value, a variable pointing to a column of one
of the relations in the range table, a parameter, or an expression
tree made of function calls, constants, variables, operators, etc.
- the qualification
The query's qualification is an expression much like one of
those contained in the target list entries. The result value of
this expression is a Boolean that tells whether the operation
(INSERT, UPDATE,
DELETE, or SELECT) for the
final result row should be executed or not. It corresponds to the WHERE clause
of an SQL statement.
- the join tree
The query's join tree shows the structure of the FROM clause.
For a simple query like SELECT ... FROM a, b, c, the join tree is just
a list of the FROM items, because we are allowed to join them in
any order. But when JOIN expressions, particularly outer joins,
are used, we have to join in the order shown by the joins.
In that case, the join tree shows the structure of the JOIN expressions. The
restrictions associated with particular JOIN clauses (from ON or
USING expressions) are stored as qualification expressions attached
to those join-tree nodes. It turns out to be convenient to store
the top-level WHERE expression as a qualification attached to the
top-level join-tree item, too. So really the join tree represents
both the FROM and WHERE clauses of a SELECT.
- the others
The other parts of the query tree like the ORDER BY
clause aren't of interest here. The rule system
substitutes some entries there while applying rules, but that
doesn't have much to do with the fundamentals of the rule
system.
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