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><H1
><A
NAME="SQL-PREPARE"
></A
>PREPARE</H1
><DIV
CLASS="REFNAMEDIV"
><A
NAME="AEN58649"
></A
><H2
>Name</H2
>PREPARE -- prepare a statement for execution</DIV
><A
NAME="AEN58652"
></A
><A
NAME="AEN58654"
></A
><DIV
CLASS="REFSYNOPSISDIV"
><A
NAME="AEN58657"
></A
><H2
>Synopsis</H2
><PRE
CLASS="SYNOPSIS"
>PREPARE <TT
CLASS="REPLACEABLE"
><I
>name</I
></TT
> [ ( <TT
CLASS="REPLACEABLE"
><I
>datatype</I
></TT
> [, ...] ) ] AS <TT
CLASS="REPLACEABLE"
><I
>statement</I
></TT
></PRE
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN58662"
></A
><H2
>Description</H2
><P
> <TT
CLASS="COMMAND"
>PREPARE</TT
> creates a prepared statement. A prepared
statement is a server-side object that can be used to optimize
performance. When the <TT
CLASS="COMMAND"
>PREPARE</TT
> statement is
executed, the specified statement is parsed, rewritten, and
planned. When an <TT
CLASS="COMMAND"
>EXECUTE</TT
> command is subsequently
issued, the prepared statement need only be executed. Thus, the
parsing, rewriting, and planning stages are only performed once,
instead of every time the statement is executed.
</P
><P
> Prepared statements can take parameters: values that are
substituted into the statement when it is executed. When creating
the prepared statement, refer to parameters by position, using
<TT
CLASS="LITERAL"
>$1</TT
>, <TT
CLASS="LITERAL"
>$2</TT
>, etc. A corresponding list of
parameter data types can optionally be specified. When a
parameter's data type is not specified or is declared as
<TT
CLASS="LITERAL"
>unknown</TT
>, the type is inferred from the context
in which the parameter is used (if possible). When executing the
statement, specify the actual values for these parameters in the
<TT
CLASS="COMMAND"
>EXECUTE</TT
> statement. Refer to <A
HREF="sql-execute.html"
><I
>EXECUTE</I
></A
> for more
information about that.
</P
><P
> Prepared statements only last for the duration of the current
database session. When the session ends, the prepared statement is
forgotten, so it must be recreated before being used again. This
also means that a single prepared statement cannot be used by
multiple simultaneous database clients; however, each client can create
their own prepared statement to use. The prepared statement can be
manually cleaned up using the <A
HREF="sql-deallocate.html"
><I
>DEALLOCATE</I
></A
> command.
</P
><P
> Prepared statements have the largest performance advantage when a
single session is being used to execute a large number of similar
statements. The performance difference will be particularly
significant if the statements are complex to plan or rewrite, for
example, if the query involves a join of many tables or requires
the application of several rules. If the statement is relatively simple
to plan and rewrite but relatively expensive to execute, the
performance advantage of prepared statements will be less noticeable.
</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN58677"
></A
><H2
>Parameters</H2
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><TT
CLASS="REPLACEABLE"
><I
>name</I
></TT
></DT
><DD
><P
> An arbitrary name given to this particular prepared
statement. It must be unique within a single session and is
subsequently used to execute or deallocate a previously prepared
statement.
</P
></DD
><DT
><TT
CLASS="REPLACEABLE"
><I
>datatype</I
></TT
></DT
><DD
><P
> The data type of a parameter to the prepared statement. If the
data type of a particular parameter is unspecified or is
specified as <TT
CLASS="LITERAL"
>unknown</TT
>, it will be inferred
from the context in which the parameter is used. To refer to the
parameters in the prepared statement itself, use
<TT
CLASS="LITERAL"
>$1</TT
>, <TT
CLASS="LITERAL"
>$2</TT
>, etc.
</P
></DD
><DT
><TT
CLASS="REPLACEABLE"
><I
>statement</I
></TT
></DT
><DD
><P
> Any <TT
CLASS="COMMAND"
>SELECT</TT
>, <TT
CLASS="COMMAND"
>INSERT</TT
>, <TT
CLASS="COMMAND"
>UPDATE</TT
>,
<TT
CLASS="COMMAND"
>DELETE</TT
>, or <TT
CLASS="COMMAND"
>VALUES</TT
> statement.
</P
></DD
></DL
></DIV
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN58703"
></A
><H2
>Notes</H2
><P
> In some situations, the query plan produced for a prepared
statement will be inferior to the query plan that would have been
chosen if the statement had been submitted and executed
normally. This is because when the statement is planned and the
planner attempts to determine the optimal query plan, the actual
values of any parameters specified in the statement are
unavailable. <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> collects
statistics on the distribution of data in the table, and can use
constant values in a statement to make guesses about the likely
result of executing the statement. Since this data is unavailable
when planning prepared statements with parameters, the chosen plan
might be suboptimal. To examine the query plan
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> has chosen for a prepared
statement, use <A
HREF="sql-explain.html"
><I
>EXPLAIN</I
></A
>.
</P
><P
> For more information on query planning and the statistics collected
by <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> for that purpose, see
the <A
HREF="sql-analyze.html"
><I
>ANALYZE</I
></A
>
documentation.
</P
><P
> You can see all available prepared statements of a session by querying the
<A
HREF="view-pg-prepared-statements.html"
><TT
CLASS="STRUCTNAME"
>pg_prepared_statements</TT
></A
>
system view.
</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="SQL-PREPARE-EXAMPLES"
></A
><H2
>Examples</H2
><P
> Create a prepared statement for an <TT
CLASS="COMMAND"
>INSERT</TT
>
statement, and then execute it:
</P><PRE
CLASS="PROGRAMLISTING"
>PREPARE fooplan (int, text, bool, numeric) AS
INSERT INTO foo VALUES($1, $2, $3, $4);
EXECUTE fooplan(1, 'Hunter Valley', 't', 200.00);</PRE
><P>
</P
><P
> Create a prepared statement for a <TT
CLASS="COMMAND"
>SELECT</TT
>
statement, and then execute it:
</P><PRE
CLASS="PROGRAMLISTING"
>PREPARE usrrptplan (int) AS
SELECT * FROM users u, logs l WHERE u.usrid=$1 AND u.usrid=l.usrid
AND l.date = $2;
EXECUTE usrrptplan(1, current_date);</PRE
><P>
Note that the data type of the second parameter is not specified,
so it is inferred from the context in which <TT
CLASS="LITERAL"
>$2</TT
> is used.
</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN58724"
></A
><H2
>Compatibility</H2
><P
> The SQL standard includes a <TT
CLASS="COMMAND"
>PREPARE</TT
> statement,
but it is only for use in embedded SQL. This version of the
<TT
CLASS="COMMAND"
>PREPARE</TT
> statement also uses a somewhat different
syntax.
</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN58729"
></A
><H2
>See Also</H2
><A
HREF="sql-deallocate.html"
><I
>DEALLOCATE</I
></A
>, <A
HREF="sql-execute.html"
><I
>EXECUTE</I
></A
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>
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