- A less obvious use is to abbreviate a normal
- SELECT from tables:
+ Note that if a FROM clause is not specified,
+ the query cannot reference any database tables. For example, the
+ following query is invalid:
SELECT distributors.* WHERE distributors.name = 'Westward';
-
- did | name
------+----------
- 108 | Westward
-
- This works because an implicit FROM item is
- added for each table that is referenced in other parts of the
- SELECT statement but not mentioned in
- FROM.
-
-
- While this is a convenient shorthand, it's easy to misuse. For
- example, the command
-SELECT distributors.* FROM distributors d;
-
- is probably a mistake; most likely the user meant
-SELECT d.* FROM distributors d;
-
- rather than the unconstrained join
-SELECT distributors.* FROM distributors d, distributors distributors;
- that he will actually get. To help detect this sort of mistake,
-
PostgreSQL will warn if the
- implicit-FROM feature is used in a
- SELECT statement that also contains an explicit
- FROM clause. Also, it is possible to disable
- the implicit-FROM feature by setting the
- parameter to false.
+
PostgreSQL releases prior to
+ 8.1 would accept queries of this form, and add an implicit entry
+ to the query's FROM clause for each table
+ referenced by the query. This is no longer the default behavior,
+ because it does not comply with the SQL standard, and is
+ considered by many to be error-prone. For compatibility with
+ applications that rely on this behavior the
+ linkend="guc-add-missing-from"> configuration variable can be
+ enabled.
SHOW ALL;
name | setting
--------------------------------+----------------------------------------------
- add_missing_from | on
+ add_missing_from | off
archive_command | unset
australian_timezones | off
.
- When true, tables that are referenced by a query will be
- automatically added to the FROM clause if not already
- present. The default is true for compatibility with
- previous releases of
PostgreSQL. However, this- behavior is not SQL-standard, and many people dislike it because it
- can mask mistakes (such as referencing a table where you should have
- referenced its alias). Set to false for the SQL-standard
- behavior of rejecting references to tables that are not listed in
- FROM.
+ When true, tables that are referenced by a query
+ will be automatically added to the FROM clause if
+ not already present. This behavior does not comply with the
+ SQL standard and many people dislike it because it can mask
+ mistakes (such as referencing a table where you should have
+ referenced its alias). The default is false. This
+ variable can be enabled for compatibility with releases of
+
PostgreSQL prior to 8.1, where this behavior+ was allowed by default.
+
+
+ Note that even when this variable is enabled, a warning
+ message will be emitted for each implicit FROM
+ entry referenced by a query. Users are encouraged to update
+ their applications to not rely on this behavior, by adding all
+ tables referenced by a query to the query's FROM
+ clause (or its USING clause in the case of
+ DELETE).
* Written by Peter Eisentraut
. *
* IDENTIFICATION
- * $PostgreSQL: pgsql/src/backend/utils/misc/guc.c,v 1.257 2005/03/25 16:17:27 tgl Exp $
+ * $PostgreSQL: pgsql/src/backend/utils/misc/guc.c,v 1.258 2005/04/08 00:59:59 neilc Exp $
*
*--------------------------------------------------------------------
*/
NULL
},
&add_missing_from,
- true, NULL, NULL
+ false, NULL, NULL
},
{
{"check_function_bodies", PGC_USERSET, CLIENT_CONN_STATEMENT,
# - Previous Postgres Versions -
-#add_missing_from = true
+#add_missing_from = false
#regex_flavor = advanced # advanced, extended, or basic
#sql_inheritance = true
#default_with_oids = false
projects / postgresql.git / commitdiff