PQspecPrepare(3) Prepares a libpqtypes specifier format string.


#include <libpqtypes.h>

int PQspecPrepare(PGconn *conn, const char *name,

                      const char *format, int is_stmt);
void PQclearSpecs(PGconn *conn);


PQspecPrepare allows an application to prepare specifier format strings that will be used frequently. By preparing a specifier format string, one avoids the parsing and type handler lookup costs. This becomes a significant win when managing large result sets or arrays, where the specifier format, like "%int4 %text %bytea", must be prepared for each tuple/element.

As with PQregisterXXX, only specifier format strings prepared prior to the creation of a PGresult or PGparam, will be available for use. This is because the prepared type spec is cached within a PGconn object and copied to all subsequent PGparam and PGresult objects.

Every prepared type spec is given a name, which is used as its unique identifier. To use a prepared type spec, the name is provided where ever a regular specifier format string is allowed, like PQputf or PQgetf. The name must be proceeded by a '@' AT sign. For more information about the syntax, see the pqt-specs(3) man page.

The format argument is the specifier format string being prepared. When this is NULL, the name prepared type spec is removed from the PGconn's internal array.

The is_stmt argument indicates if a parementerized statement version of format should be cached along with the prepared type spec. This means all type specifiers in format, like "%int4", will be converted to "$1" syntax. When is_stmt is non-zero, a statement will created and cached. For more information on specifer format string to paremterized statements, see the PQputf(3) man page. NOTE: to use a prepared type spec with execution functions like PQexecf, is_stmt must be set to non-zero.

PQclearSpecs removes all prepared specifiers from the given PGconn, as opposed to removing them one by one by setting PQspecPrepare's format argument to NULL. A good use for this is after a PQresetXXX call when it might be desired to re-prepare all type specifiers.

Functions that support the use of a prepared type spec are: PQputf, PQputvf, PQgetf, PQgetvf, PQexecf, PQexecvf, PQsendf, PQsendvf, PQparamExec, PQparamSendQuery.

HINT: A good rule of thumb for using prepared type specs, is when there are a large number of PQputf/PQgetf calls per statement execution. This commonly occurs when dealing with large result sets and arrays.


PQspecPrepare and PQclearSpecs return a nonzero value on success and zero if it fails. Use PQgeterror() to get an error message.


This example prepares a type spec and issues some PQputf calls.

int i;
PQparam *param;
if(!PQspecPrepare(conn, "prepared_spec", "%int4 %text", 0))
        fprintf(stderr, "PQspecPrepare: %s, PQgeterror());
/* create after preparing spec */
param = PQparamCreate(conn);
for(i=0; i < 100000; i++)
        /* NOTE: nothing else can be in format string */
        PQputf(param, "@prepared_spec", 4, "text");
/* This elects to prepare a statement as well.  After this returns,
 * "SELECT myfunc($1, $2)" will be cached along with the prepared spec.
PQspecPrepare(conn, "myfunc", "SELECT myfunc(%int4, %text)", 1);
/* "myfunc" tells execf to execute "SELECT myfunc($1, $2)".  If is_stmt
 * was set to zero during the PQspecPrepare, the below would be invalid
 * because execf doesn't know what to execute.
PQexecf(conn, "@myfunc", 123, "text");
/* clear'm all */




A contribution of eSilo, LLC. for the PostgreSQL Database Management System. Written by Andrew Chernow.


Report bugs to <[email protected]>.


Copyright (c) 2011 eSilo, LLC. All rights reserved.
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