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><H1
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NAME="SQL-SET-CONSTRAINTS"
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>SET CONSTRAINTS</H1
><DIV
CLASS="REFNAMEDIV"
><A
NAME="AEN78865"
></A
><H2
>Name</H2
>SET CONSTRAINTS -- set constraint check timing for the current transaction</DIV
><DIV
CLASS="REFSYNOPSISDIV"
><A
NAME="AEN78870"
></A
><H2
>Synopsis</H2
><PRE
CLASS="SYNOPSIS"
>SET CONSTRAINTS { ALL | <TT
CLASS="REPLACEABLE"
><I
>name</I
></TT
> [, ...] } { DEFERRED | IMMEDIATE }</PRE
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN78873"
></A
><H2
>Description</H2
><P
> <TT
CLASS="COMMAND"
>SET CONSTRAINTS</TT
> sets the behavior of constraint
checking within the current transaction. <TT
CLASS="LITERAL"
>IMMEDIATE</TT
>
constraints are checked at the end of each
statement. <TT
CLASS="LITERAL"
>DEFERRED</TT
> constraints are not checked until
transaction commit. Each constraint has its own
<TT
CLASS="LITERAL"
>IMMEDIATE</TT
> or <TT
CLASS="LITERAL"
>DEFERRED</TT
> mode.
</P
><P
> Upon creation, a constraint is given one of three
characteristics: <TT
CLASS="LITERAL"
>DEFERRABLE INITIALLY DEFERRED</TT
>,
<TT
CLASS="LITERAL"
>DEFERRABLE INITIALLY IMMEDIATE</TT
>, or
<TT
CLASS="LITERAL"
>NOT DEFERRABLE</TT
>. The third
class is always <TT
CLASS="LITERAL"
>IMMEDIATE</TT
> and is not affected by the
<TT
CLASS="COMMAND"
>SET CONSTRAINTS</TT
> command. The first two classes start
every transaction in the indicated mode, but their behavior can be changed
within a transaction by <TT
CLASS="COMMAND"
>SET CONSTRAINTS</TT
>.
</P
><P
> <TT
CLASS="COMMAND"
>SET CONSTRAINTS</TT
> with a list of constraint names changes
the mode of just those constraints (which must all be deferrable). Each
constraint name can be schema-qualified. The
current schema search path is used to find the first matching name if
no schema name is specified. <TT
CLASS="COMMAND"
>SET CONSTRAINTS ALL</TT
>
changes the mode of all deferrable constraints.
</P
><P
> When <TT
CLASS="COMMAND"
>SET CONSTRAINTS</TT
> changes the mode of a constraint
from <TT
CLASS="LITERAL"
>DEFERRED</TT
>
to <TT
CLASS="LITERAL"
>IMMEDIATE</TT
>, the new mode takes effect
retroactively: any outstanding data modifications that would have
been checked at the end of the transaction are instead checked during the
execution of the <TT
CLASS="COMMAND"
>SET CONSTRAINTS</TT
> command.
If any such constraint is violated, the <TT
CLASS="COMMAND"
>SET CONSTRAINTS</TT
>
fails (and does not change the constraint mode). Thus, <TT
CLASS="COMMAND"
>SET
CONSTRAINTS</TT
> can be used to force checking of constraints to
occur at a specific point in a transaction.
</P
><P
> Currently, only <TT
CLASS="LITERAL"
>UNIQUE</TT
>, <TT
CLASS="LITERAL"
>PRIMARY KEY</TT
>,
<TT
CLASS="LITERAL"
>REFERENCES</TT
> (foreign key), and <TT
CLASS="LITERAL"
>EXCLUDE</TT
>
constraints are affected by this setting.
<TT
CLASS="LITERAL"
>NOT NULL</TT
> and <TT
CLASS="LITERAL"
>CHECK</TT
> constraints are
always checked immediately when a row is inserted or modified
(<SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>not</I
></SPAN
> at the end of the statement).
Uniqueness and exclusion constraints that have not been declared
<TT
CLASS="LITERAL"
>DEFERRABLE</TT
> are also checked immediately.
</P
><P
> The firing of triggers that are declared as <SPAN
CLASS="QUOTE"
>"constraint triggers"</SPAN
>
is also controlled by this setting — they fire at the same time
that the associated constraint should be checked.
</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN78909"
></A
><H2
>Notes</H2
><P
> Because <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> does not require constraint
names to be unique within a schema (but only per-table), it is possible
that there is more than one match for a specified constraint name.
In this case <TT
CLASS="COMMAND"
>SET CONSTRAINTS</TT
> will act on all matches.
For a non-schema-qualified name, once a match or matches have been found in
some schema in the search path, schemas appearing later in the path are not
searched.
</P
><P
> This command only alters the behavior of constraints within the
current transaction. Thus, if you execute this command outside of a
transaction block
(<TT
CLASS="COMMAND"
>BEGIN</TT
>/<TT
CLASS="COMMAND"
>COMMIT</TT
> pair), it will
not appear to have any effect.
</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN78917"
></A
><H2
>Compatibility</H2
><P
> This command complies with the behavior defined in the SQL
standard, except for the limitation that, in
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>, it does not apply to
<TT
CLASS="LITERAL"
>NOT NULL</TT
> and <TT
CLASS="LITERAL"
>CHECK</TT
> constraints.
Also, <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> checks non-deferrable
uniqueness constraints immediately, not at end of statement as the
standard would suggest.
</P
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