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>PostgreSQL 8.4.20 Documentation</TH
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><DIV
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><H1
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><A
NAME="RUNTIME-CONFIG-COMPATIBLE"
>18.12. Version and Platform Compatibility</A
></H1
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="RUNTIME-CONFIG-COMPATIBLE-VERSION"
>18.12.1. Previous PostgreSQL Versions</A
></H2
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><A
NAME="GUC-ADD-MISSING-FROM"
></A
><TT
CLASS="VARNAME"
>add_missing_from</TT
> (<TT
CLASS="TYPE"
>boolean</TT
>)</DT
><DD
><P
> When on, tables that are referenced by a query will be
automatically added to the <TT
CLASS="LITERAL"
>FROM</TT
> 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 <TT
CLASS="LITERAL"
>off</TT
>. This variable can be
enabled for compatibility with releases of
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> prior to 8.1, where this behavior was
allowed by default.
</P
><P
> Note that even when this variable is enabled, a warning
message will be emitted for each implicit <TT
CLASS="LITERAL"
>FROM</TT
>
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 <TT
CLASS="LITERAL"
>FROM</TT
>
clause (or its <TT
CLASS="LITERAL"
>USING</TT
> clause in the case of
<TT
CLASS="COMMAND"
>DELETE</TT
>).
</P
></DD
><DT
><A
NAME="GUC-ARRAY-NULLS"
></A
><TT
CLASS="VARNAME"
>array_nulls</TT
> (<TT
CLASS="TYPE"
>boolean</TT
>)</DT
><DD
><P
> This controls whether the array input parser recognizes
unquoted <TT
CLASS="LITERAL"
>NULL</TT
> as specifying a null array element.
By default, this is <TT
CLASS="LITERAL"
>on</TT
>, allowing array values containing
null values to be entered. However, <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> versions
before 8.2 did not support null values in arrays, and therefore would
treat <TT
CLASS="LITERAL"
>NULL</TT
> as specifying a normal array element with
the string value <SPAN
CLASS="QUOTE"
>"NULL"</SPAN
>. For backwards compatibility with
applications that require the old behavior, this variable can be
turned <TT
CLASS="LITERAL"
>off</TT
>.
</P
><P
> Note that it is possible to create array values containing null values
even when this variable is <TT
CLASS="LITERAL"
>off</TT
>.
</P
></DD
><DT
><A
NAME="GUC-BACKSLASH-QUOTE"
></A
><TT
CLASS="VARNAME"
>backslash_quote</TT
> (<TT
CLASS="TYPE"
>enum</TT
>)</DT
><DD
><P
> This controls whether a quote mark can be represented by
<TT
CLASS="LITERAL"
>\'</TT
> in a string literal. The preferred, SQL-standard way
to represent a quote mark is by doubling it (<TT
CLASS="LITERAL"
>''</TT
>) but
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> has historically also accepted
<TT
CLASS="LITERAL"
>\'</TT
>. However, use of <TT
CLASS="LITERAL"
>\'</TT
> creates security risks
because in some client character set encodings, there are multibyte
characters in which the last byte is numerically equivalent to ASCII
<TT
CLASS="LITERAL"
>\</TT
>. 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 <TT
CLASS="VARNAME"
>backslash_quote</TT
> are
<TT
CLASS="LITERAL"
>on</TT
> (allow <TT
CLASS="LITERAL"
>\'</TT
> always),
<TT
CLASS="LITERAL"
>off</TT
> (reject always), and
<TT
CLASS="LITERAL"
>safe_encoding</TT
> (allow only if client encoding does not
allow ASCII <TT
CLASS="LITERAL"
>\</TT
> within a multibyte character).
<TT
CLASS="LITERAL"
>safe_encoding</TT
> is the default setting.
</P
><P
> Note that in a standard-conforming string literal, <TT
CLASS="LITERAL"
>\</TT
> just
means <TT
CLASS="LITERAL"
>\</TT
> anyway. This parameter affects the handling of
non-standard-conforming literals, including
escape string syntax (<TT
CLASS="LITERAL"
>E'...'</TT
>).
</P
></DD
><DT
><A
NAME="GUC-DEFAULT-WITH-OIDS"
></A
><TT
CLASS="VARNAME"
>default_with_oids</TT
> (<TT
CLASS="TYPE"
>boolean</TT
>)</DT
><DD
><P
> This controls whether <TT
CLASS="COMMAND"
>CREATE TABLE</TT
> and
<TT
CLASS="COMMAND"
>CREATE TABLE AS</TT
> include an OID column in
newly-created tables, if neither <TT
CLASS="LITERAL"
>WITH OIDS</TT
>
nor <TT
CLASS="LITERAL"
>WITHOUT OIDS</TT
> is specified. It also
determines whether OIDs will be included in tables created by
<TT
CLASS="COMMAND"
>SELECT INTO</TT
>. In <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>
8.1 <TT
CLASS="VARNAME"
>default_with_oids</TT
> is <TT
CLASS="LITERAL"
>off</TT
> by default; in
prior versions of <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>, it
was on by default.
</P
><P
> 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 <TT
CLASS="LITERAL"
>WITH OIDS</TT
> when creating the
table. This variable can be enabled for compatibility with old
applications that do not follow this behavior.
</P
></DD
><DT
><A
NAME="GUC-ESCAPE-STRING-WARNING"
></A
><TT
CLASS="VARNAME"
>escape_string_warning</TT
> (<TT
CLASS="TYPE"
>boolean</TT
>)</DT
><DD
><P
> When on, a warning is issued if a backslash (<TT
CLASS="LITERAL"
>\</TT
>)
appears in an ordinary string literal (<TT
CLASS="LITERAL"
>'...'</TT
>
syntax) and <TT
CLASS="VARNAME"
>standard_conforming_strings</TT
> is off.
The default is <TT
CLASS="LITERAL"
>on</TT
>.
</P
><P
> Applications that wish to use backslash as escape should be
modified to use escape string syntax (<TT
CLASS="LITERAL"
>E'...'</TT
>),
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.
</P
></DD
><DT
><A
NAME="GUC-REGEX-FLAVOR"
></A
><TT
CLASS="VARNAME"
>regex_flavor</TT
> (<TT
CLASS="TYPE"
>enum</TT
>)</DT
><DD
><P
> The regular expression <SPAN
CLASS="QUOTE"
>"flavor"</SPAN
> can be set to
<TT
CLASS="LITERAL"
>advanced</TT
>, <TT
CLASS="LITERAL"
>extended</TT
>, or <TT
CLASS="LITERAL"
>basic</TT
>.
The default is <TT
CLASS="LITERAL"
>advanced</TT
>. The <TT
CLASS="LITERAL"
>extended</TT
>
setting might be useful for exact backwards compatibility with
pre-7.4 releases of <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>. See
<A
HREF="functions-matching.html#POSIX-SYNTAX-DETAILS"
>Section 9.7.3.1</A
> for details.
</P
></DD
><DT
><A
NAME="GUC-SQL-INHERITANCE"
></A
><TT
CLASS="VARNAME"
>sql_inheritance</TT
> (<TT
CLASS="TYPE"
>boolean</TT
>)</DT
><DD
><P
> This setting controls whether undecorated table references are
considered to include inheritance child tables. The default is
<TT
CLASS="LITERAL"
>on</TT
>, which means child tables are included (thus,
a <TT
CLASS="LITERAL"
>*</TT
> suffix is assumed by default). If turned
<TT
CLASS="LITERAL"
>off</TT
>, child tables are not included (thus, an
<TT
CLASS="LITERAL"
>ONLY</TT
> prefix is assumed). The SQL standard
requires child tables to be included, so the <TT
CLASS="LITERAL"
>off</TT
> setting
is not spec-compliant, but it is provided for compatibility with
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> releases prior to 7.1.
See <A
HREF="ddl-inherit.html"
>Section 5.8</A
> for more information.
</P
><P
> Turning <TT
CLASS="VARNAME"
>sql_inheritance</TT
> off is deprecated, because that
behavior has been found to be error-prone as well as contrary to SQL
standard. Discussions of inheritance behavior elsewhere in this
manual generally assume that it is <TT
CLASS="LITERAL"
>on</TT
>.
</P
></DD
><DT
><A
NAME="GUC-STANDARD-CONFORMING-STRINGS"
></A
><TT
CLASS="VARNAME"
>standard_conforming_strings</TT
> (<TT
CLASS="TYPE"
>boolean</TT
>)</DT
><DD
><P
> This controls whether ordinary string literals
(<TT
CLASS="LITERAL"
>'...'</TT
>) treat backslashes literally, as specified in
the SQL standard.
The default is currently <TT
CLASS="LITERAL"
>off</TT
>, causing
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> to have its historical
behavior of treating backslashes as escape characters.
The default will change to <TT
CLASS="LITERAL"
>on</TT
> in a future release
to improve compatibility with the standard.
Applications can 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 (<TT
CLASS="LITERAL"
>E'...'</TT
>) is supported.
Escape string syntax should be used if an application desires
backslashes to be treated as escape characters.
</P
></DD
><DT
><A
NAME="GUC-SYNCHRONIZE-SEQSCANS"
></A
><TT
CLASS="VARNAME"
>synchronize_seqscans</TT
> (<TT
CLASS="TYPE"
>boolean</TT
>)</DT
><DD
><P
> This allows sequential scans of large tables to synchronize with each
other, so that concurrent scans read the same block at about the
same time and hence share the I/O workload. When this is enabled,
a scan might start in the middle of the table and then <SPAN
CLASS="QUOTE"
>"wrap
around"</SPAN
> the end to cover all rows, so as to synchronize with the
activity of scans already in progress. This can result in
unpredictable changes in the row ordering returned by queries that
have no <TT
CLASS="LITERAL"
>ORDER BY</TT
> clause. Setting this parameter to
<TT
CLASS="LITERAL"
>off</TT
> ensures the pre-8.3 behavior in which a sequential
scan always starts from the beginning of the table. The default
is <TT
CLASS="LITERAL"
>on</TT
>.
</P
></DD
></DL
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="RUNTIME-CONFIG-COMPATIBLE-CLIENTS"
>18.12.2. Platform and Client Compatibility</A
></H2
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><A
NAME="GUC-TRANSFORM-NULL-EQUALS"
></A
><TT
CLASS="VARNAME"
>transform_null_equals</TT
> (<TT
CLASS="TYPE"
>boolean</TT
>)</DT
><DD
><P
> When on, expressions of the form <TT
CLASS="LITERAL"
><TT
CLASS="REPLACEABLE"
><I
>expr</I
></TT
> =
NULL</TT
> (or <TT
CLASS="LITERAL"
>NULL =
<TT
CLASS="REPLACEABLE"
><I
>expr</I
></TT
></TT
>) are treated as
<TT
CLASS="LITERAL"
><TT
CLASS="REPLACEABLE"
><I
>expr</I
></TT
> IS NULL</TT
>, that is, they
return true if <TT
CLASS="REPLACEABLE"
><I
>expr</I
></TT
> evaluates to the null value,
and false otherwise. The correct SQL-spec-compliant behavior of
<TT
CLASS="LITERAL"
><TT
CLASS="REPLACEABLE"
><I
>expr</I
></TT
> = NULL</TT
> is to always
return null (unknown). Therefore this parameter defaults to
<TT
CLASS="LITERAL"
>off</TT
>.
</P
><P
> However, filtered forms in <SPAN
CLASS="PRODUCTNAME"
>Microsoft
Access</SPAN
> generate queries that appear to use
<TT
CLASS="LITERAL"
><TT
CLASS="REPLACEABLE"
><I
>expr</I
></TT
> = NULL</TT
> 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 <TT
CLASS="LITERAL"
><TT
CLASS="REPLACEABLE"
><I
>expr</I
></TT
> = NULL</TT
> 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.
</P
><P
> Note that this option only affects the exact form <TT
CLASS="LITERAL"
>= NULL</TT
>,
not other comparison operators or other expressions
that are computationally equivalent to some expression
involving the equals operator (such as <TT
CLASS="LITERAL"
>IN</TT
>).
Thus, this option is not a general fix for bad programming.
</P
><P
> Refer to <A
HREF="functions-comparison.html"
>Section 9.2</A
> for related information.
</P
></DD
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