add_missing_from
(boolean
)
When on, 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 off
. 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
).
array_nulls
(boolean
)
This controls whether the array input parser recognizes
unquoted NULL
as specifying a null array element.
By default, this is on
, allowing array values containing
null values to be entered. However, PostgreSQL versions
before 8.2 did not support null values in arrays, and therefore would
treat NULL
as specifying a normal array element with
the string value “NULL”. For backwards compatibility with
applications that require the old behavior, this variable can be
turned off
.
Note that it is possible to create array values containing null values
even when this variable is off
.
backslash_quote
(string
)
This controls whether a quote mark can be represented by
\'
in a string literal. The preferred, SQL-standard way
to represent a quote mark is by doubling it (''
) but
PostgreSQL has historically also accepted
\'
. However, use of \'
creates security risks
because in some client character set encodings, there are multibyte
characters in which the last byte is numerically equivalent to ASCII
\
. If client-side code does escaping incorrectly then a
SQL-injection attack is possible. This risk can be prevented by
making the server reject queries in which a quote mark appears to be
escaped by a backslash.
The allowed values of backslash_quote
are
on
(allow \'
always),
off
(reject always), and
safe_encoding
(allow only if client encoding does not
allow ASCII \
within a multibyte character).
safe_encoding
is the default setting.
Note that in a standard-conforming string literal, \
just
means \
anyway. This parameter affects the handling of
non-standard-conforming literals, including
escape string syntax (E'...'
).
default_with_oids
(boolean
)
This controls whether CREATE TABLE
and
CREATE TABLE AS
include an OID column in
newly-created tables, if neither WITH OIDS
nor WITHOUT OIDS
is specified. It also
determines whether OIDs will be included in tables created by
SELECT INTO
. In PostgreSQL
8.1 default_with_oids
is off
by default; in
prior versions of PostgreSQL, it
was on by default.
The use of OIDs in user tables is considered deprecated, so
most installations should leave this variable disabled.
Applications that require OIDs for a particular table should
specify WITH OIDS
when creating the
table. This variable can be enabled for compatibility with old
applications that do not follow this behavior.
escape_string_warning
(boolean
)
When on, a warning is issued if a backslash (\
)
appears in an ordinary string literal ('...'
syntax) and standard_conforming_strings
is off.
The default is on
.
Applications that wish to use backslash as escape should be
modified to use escape string syntax (E'...'
),
because the default behavior of ordinary strings will change
in a future release for SQL compatibility. This variable can
be enabled to help detect applications that will break.
regex_flavor
(string
)
The regular expression “flavor” can be set to
advanced
, extended
, or basic
.
The default is advanced
. The extended
setting may be useful for exact backwards compatibility with
pre-7.4 releases of PostgreSQL. See
Section 9.7.3.1, “Regular Expression Details” for details.
sql_inheritance
(boolean
)
This controls the inheritance semantics. If turned off
,
subtables are not included by various commands by default; basically
an implied ONLY
key word. This was added for
compatibility with releases prior to 7.1. See
Section 5.8, “Inheritance” for more information.
standard_conforming_strings
(boolean
)
This controls whether ordinary string literals
('...'
) treat backslashes literally, as specified in
the SQL standard.
The default is currently off
, causing
PostgreSQL to have its historical
behavior of treating backslashes as escape characters.
The default will change to on
in a future release
to improve compatibility with the standard.
Applications may check this
parameter to determine how string literals will be processed.
The presence of this parameter can also be taken as an indication
that the escape string syntax (E'...'
) is supported.
Escape string syntax should be used if an application desires
backslashes to be treated as escape characters.
transform_null_equals
(boolean
)
When on, expressions of the form
(or expr
=
NULLNULL =
) are treated as
expr
, that is, they
return true if expr
IS NULLexpr
evaluates to the null value,
and false otherwise. The correct SQL-spec-compliant behavior of
is to always
return null (unknown). Therefore this parameter defaults to
expr
= NULLoff
.
However, filtered forms in Microsoft
Access generate queries that appear to use
to test for
null values, so if you use that interface to access the database you
might want to turn this option on. Since expressions of the
form expr
= NULL
always
return the null value (using the correct interpretation) they are not
very useful and do not appear often in normal applications, so
this option does little harm in practice. But new users are
frequently confused about the semantics of expressions
involving null values, so this option is not on by default.
expr
= NULL
Note that this option only affects the exact form = NULL
,
not other comparison operators or other expressions
that are computationally equivalent to some expression
involving the equals operator (such as IN
).
Thus, this option is not a general fix for bad programming.
Refer to Section 9.2, “Comparison Operators” for related information.