soci/docs/backends/postgresql.md

PostgreSQL Backend Reference

SOCI backend for accessing PostgreSQL database.

Prerequisites

Supported Versions

The SOCI PostgreSQL backend is supported for use with PostgreSQL >= 7.3, although versions older than 8.0 will suffer from limited feature support. See below for details.

Tested Platforms

PostgreSQL version	Operating System	Compiler
9.0	Mac OS X 10.6.6	g++ 4.2
8.4	FreeBSD 8.2	g++ 4.1
8.4	Debian 6	g++ 4.3
8.4	RedHat 5	g++ 4.3

Required Client Libraries

The SOCI PostgreSQL backend requires PostgreSQL's libpq client library.

Note that the SOCI library itself depends also on libdl, so the minimum set of libraries needed to compile a basic client program is:

-lsoci_core -lsoci_postgresql -ldl -lpq

Connecting to the Database

To establish a connection to the PostgreSQL database, create a session object using the postgresql backend factory together with a connection string:

session sql(postgresql, "dbname=mydatabase");

// or:
session sql("postgresql", "dbname=mydatabase");

// or:
session sql("postgresql://dbname=mydatabase");

The set of parameters used in the connection string for PostgreSQL is the same as accepted by the [PQconnectdb](http://www.postgresql.org/docs/8.3/interactive/libpq.html#LIBPQ-CONNECT) function from the libpq library.

In addition to standard PostgreSQL connection parameters, the following can be set:

• singlerow or singlerows

For example:

session sql(postgresql, "dbname=mydatabase singlerows=true");

If the singlerows parameter is set to true or yes, then queries will be executed in the single-row mode, which prevents the client library from loading full query result set into memory and instead fetches rows one by one, as they are requested by the statement's fetch() function. This mode can be of interest to those users who want to make their client applications more responsive (with more fine-grained operation) by avoiding potentially long blocking times when complete query results are loaded to client's memory. Note that in the single-row operation:

• bulk queries are not supported, and
• in order to fulfill the expectations of the underlying client library, the complete rowset has to be exhausted before executing further queries on the same session.

Once you have created a session object as shown above, you can use it to access the database, for example:

int count;
sql << "select count(*) from invoices", into(count);

(See the [exchanging data](../basics.html">SOCI basics and <a href="../exchange.html) documentation for general information on using the session class.)

SOCI Feature Support

Dynamic Binding

The PostgreSQL backend supports the use of the SOCI row class, which facilitates retrieval of data whose type is not known at compile time.

When calling row::get<T>(), the type you should pass as T depends upon the underlying database type.
For the PostgreSQL backend, this type mapping is:

PostgreSQL Data Type	SOCI Data Type	row::get<T> specializations
numeric, real, double	dt_double	double
boolean, smallint, integer	dt_integer	int
int8	dt_long_long	long long
oid	dt_integer	unsigned long
char, varchar, text, cstring, bpchar	dt_string	std::string
abstime, reltime, date, time, timestamp, timestamptz, timetz	dt_date	std::tm

(See the dynamic resultset binding documentation for general information on using the row class.)

Binding by Name

In addition to binding by position, the PostgreSQL backend supports binding by name, via an overload of the use() function:

int id = 7;
sql << "select name from person where id = :id", use(id, "id")

Apart from the portable "colon-name" syntax above, which is achieved by rewriting the query string, the backend also supports the PostgreSQL native numbered syntax:

int i = 7;
int j = 8;
sql << "insert into t(x, y) values($1, $2)", use(i), use(j);

The use of native syntax is not recommended, but can be nevertheless imposed by switching off the query rewriting. This can be achieved by defining the macro SOCI_POSTGRESQL_NOBINDBYNAME and it is actually necessary for PostgreSQL 7.3, in which case binding of use elements is not supported at all. See the Configuration options section for details.

Bulk Operations

The PostgreSQL backend has full support for SOCI's bulk operations interface.

Transactions

Transactions are also fully supported by the PostgreSQL backend.

blob Data Type

The PostgreSQL backend supports working with data stored in columns of type Blob, via SOCI's blob class with the exception that trimming is not supported.

rowid Data Type

The concept of row identifier (OID in PostgreSQL) is supported via SOCI's rowid class.

Nested Statements

Nested statements are not supported by PostgreSQL backend.

Stored Procedures

PostgreSQL stored procedures can be executed by using SOCI's procedure class.

Native API Access

SOCI provides access to underlying datbabase APIs via several get_backend() functions, as described in the beyond SOCI documentation.

The PostgreSQL backend provides the following concrete classes for navite API access:

Accessor Function	Concrete Class
session_backend * session::get_backend()	postgresql_session_backend
statement_backend * statement::get_backend()	postgresql_statement_backend
blob_backend * blob::get_backend()	postgresql_blob_backend
rowid_backend * rowid::get_backend()	postgresql_rowid_backend

Backend-specific extensions

uuid Data Type

The PostgreSQL backend supports working with data stored in columns of type UUID via simple string operations. All string representations of UUID supported by PostgreSQL are accepted on input, the backend will return the standard format of UUID on output. See the test test_uuid_column_type_support for usage examples.

Configuration options

To support older PostgreSQL versions, the following configuration macros are recognized:

• SOCI_POSTGRESQL_NOBINDBYNAME - switches off the query rewriting.
• SOCI_POSTGRESQL_NOPARAMS - disables support for parameterized queries (binding of use elements),automatically imposes also the SOCI_POSTGRESQL_NOBINDBYNAME macro. It is necessary for PostgreSQL 7.3.
• SOCI_POSTGRESQL_NOPREPARE - disables support for separate query preparation, which in this backend is significant only in terms of optimization. It is necessary for PostgreSQL 7.3 and 7.4.