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><H1
><A
NAME="SQL-PREPARE"
></A
>PREPARE</H1
><DIV
CLASS="REFNAMEDIV"
><A
NAME="AEN79065"
></A
><H2
>Name</H2
>PREPARE -- prepare a statement for execution</DIV
><DIV
CLASS="REFSYNOPSISDIV"
><A
NAME="AEN79073"
></A
><H2
>Synopsis</H2
><PRE
CLASS="SYNOPSIS"
>PREPARE <TT
CLASS="REPLACEABLE"
><I
>name</I
></TT
> [ ( <TT
CLASS="REPLACEABLE"
><I
>data_type</I
></TT
> [, ...] ) ] AS <TT
CLASS="REPLACEABLE"
><I
>statement</I
></TT
></PRE
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN79078"
></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, analyzed, and rewritten.
When an <TT
CLASS="COMMAND"
>EXECUTE</TT
> command is subsequently
issued, the prepared statement is planned and executed. This division
of labor avoids repetitive parse analysis work, while allowing
the execution plan to depend on the specific parameter values supplied.
</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"
>EXECUTE</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. Prepared statements can be
manually cleaned up using the <A
HREF="sql-deallocate.html"
>DEALLOCATE</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="AEN79093"
></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
>data_type</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"
>$2</TT
>, etc.
</P
></DD
><DT
><TT
CLASS="REPLACEABLE"
><I
>statement</I
></TT
></DT
><DD
><P
> Any <TT
CLASS="COMMAND"
>SELECT</TT
>, <TT
CLASS="COMMAND"
>INSERT</TT
>, <TT
CLASS="COMMAND"
>UPDATE</TT
>,
<TT
CLASS="COMMAND"
>DELETE</TT
>, or <TT
CLASS="COMMAND"
>VALUES</TT
> statement.
</P
></DD
></DL
></DIV
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN79119"
></A
><H2
>Notes</H2
><P
> If a prepared statement is executed enough times, the server may eventually
decide to save and re-use a generic plan rather than re-planning each time.
This will occur immediately if the prepared statement has no parameters;
otherwise it occurs only if the generic plan appears to be not much more
expensive than a plan that depends on specific parameter values.
Typically, a generic plan will be selected only if the query's performance
is estimated to be fairly insensitive to the specific parameter values
supplied.
</P
><P
> To examine the query plan <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> is using
for a prepared statement, use <A
HREF="sql-explain.html"
>EXPLAIN</A
>.
If a generic plan is in use, it will contain parameter symbols
<TT
CLASS="LITERAL"
>$<TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
></TT
>, while a custom plan will have the
current actual parameter values substituted into it.
</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"
>ANALYZE</A
>
documentation.
</P
><P
> Although the main point of a prepared statement is to avoid repeated parse
analysis and planning of the statement, <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> will
force re-analysis and re-planning of the statement before using it
whenever database objects used in the statement have undergone
definitional (DDL) changes since the previous use of the prepared
statement. Also, if the value of <A
HREF="runtime-config-client.html#GUC-SEARCH-PATH"
>search_path</A
> changes
from one use to the next, the statement will be re-parsed using the new
<TT
CLASS="VARNAME"
>search_path</TT
>. (This latter behavior is new as of
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> 9.3.) These rules make use of a
prepared statement semantically almost equivalent to re-submitting the
same query text over and over, but with a performance benefit if no object
definitions are changed, especially if the best plan remains the same
across uses. An example of a case where the semantic equivalence is not
perfect is that if the statement refers to a table by an unqualified name,
and then a new table of the same name is created in a schema appearing
earlier in the <TT
CLASS="VARNAME"
>search_path</TT
>, no automatic re-parse will occur
since no object used in the statement changed. However, if some other
change forces a re-parse, the new table will be referenced in subsequent
uses.
</P
><P
> You can see all prepared statements available in the 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="AEN79148"
></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="AEN79153"
></A
><H2
>See Also</H2
><A
HREF="sql-deallocate.html"
>DEALLOCATE</A
>, <A
HREF="sql-execute.html"
>EXECUTE</A
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