|The PostgreSQL 9.0 Reference Manual - Volume 1A - SQL Language Reference
by The PostgreSQL Global Development Group
Paperback (6"x9"), 454 pages
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3.7.3 The Schema Search Path
Qualified names are tedious to write, and it's often best not to wire a particular schema name into applications anyway. Therefore tables are often referred to by unqualified names, which consist of just the table name. The system determines which table is meant by following a search path, which is a list of schemas to look in. The first matching table in the search path is taken to be the one wanted. If there is no match in the search path, an error is reported, even if matching table names exist in other schemas in the database.
The first schema named in the search path is called the current schema.
Aside from being the first schema searched, it is also the schema in
which new tables will be created if the
command does not specify a schema name.
To show the current search path, use the following command:
In the default setup this returns:
search_path -------------- "$user",public
The first element specifies that a schema with the same name as the current user is to be searched. If no such schema exists, the entry is ignored. The second element refers to the public schema that we have seen already.
The first schema in the search path that exists is the default location for creating new objects. That is the reason that by default objects are created in the public schema. When objects are referenced in any other context without schema qualification (table modification, data modification, or query commands) the search path is traversed until a matching object is found. Therefore, in the default configuration, any unqualified access again can only refer to the public schema.
To put our new schema in the path, we use:
SET search_path TO myschema,public;
(We omit the
$user here because we have no
immediate need for it.) And then we can access the table without
DROP TABLE mytable;
myschema is the first element in
the path, new objects would by default be created in it.
We could also have written:
SET search_path TO myschema;
Then we no longer have access to the public schema without explicit qualification. There is nothing special about the public schema except that it exists by default. It can be dropped, too.
See also section 7.23 System Information Functions for other ways to manipulate the schema search path.
The search path works in the same way for data type names, function names, and operator names as it does for table names. Data type and function names can be qualified in exactly the same way as table names. If you need to write a qualified operator name in an expression, there is a special provision: you must write
This is needed to avoid syntactic ambiguity. An example is:
SELECT 3 OPERATOR(pg_catalog.+) 4;
In practice one usually relies on the search path for operators, so as not to have to write anything so ugly as that.
|ISBN 9781906966041||The PostgreSQL 9.0 Reference Manual - Volume 1A - SQL Language Reference||See the print edition|