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Server Specific Guide

PostgreSQL

SQLAPI++ allows to work with a number of SQL database servers. It provides common mechanisms to access database, and as a general rule they work for any database server. But each server has some specific features which a developer has to know in order to leverage server's unique features and avoid potential errors.

This page collects all specific information that concerns working with PostgreSQL server using SQLAPI++ Library. Full information about using SQLAPI++ see in How To and Online Documentation.

Available topics:

Connecting to a database

Transaction isolation levels

Working with Long or Lob(CLob, BLob) data

Returning output parameters

Cancelling queries

Connection, command, parameter and field options

Getting native PostgreSQL API

Getting native PostgreSQL connection related handles

Getting native PostgreSQL command related handles

Error handling

Special header files - Compile time

 

Connecting to a database

To connect to a database you should create a connection object and then connect it. A connection object is represented by SAConnection class. After the connection is created you need to connect it to PostgreSQL server using SAConnection::Connect method

void Connect( const SAString &sDBString, const SAString &sUserID, const SAString &sPassword, SAClient_t eSAClient = SA_Client_NotSpecified );

with the following parameters:

sDBString.  A string describes PostgreSQL connection optinons, The fromat is "[<server_name>]@][<database_name>][;<options>]":

  • "" or "@" - empty string or '@' character, connects to a local server.
  • "<database_name>" or "@ <database_name>" - connects to a database with the specified name on local server.
  • "<server_name>@" - connects to the specified server.
  • "<server_name>@<database_name>" - connects to a database with the specified name on the specified server.

<server_name> can be a cobination of hostname/address and port number (<server_name> = <host>[,<port>])) or unix_socket path.
<options> string used for PQsetdbLogin function pgoptions parameter (these are the server process parameters).
<database_name> string defines the database name for the connection but it can also define the client options like: "dbname=mydb connect_timeout=10"

sUserID.    A string containing a user name to use when establishing the connection.
sPassword.    A string containing a password to use when establishing the connection.
eSAClient.    Optional. One of the following values from SAClient_t enum:

  • SA_PostgreSQL_Client    PostgreSQL client.
  • SA_Client_NotSpecified     Used by default if eSAClientparameter is omitted.  You can use this default value only if you have call SAConnection::setClient method with SA_PostgreSQL_Client constant before.

The SQLAPI++ Library requires libpq version 7.1.x or higher.

For more details see How To - Connecting to databases, SAConnection object, SAConnection::Connect.

 

Transaction isolation levels

SQL-92 defines four isolation levels, all of which are supported by SQLAPI++:

  • Read uncommitted (the lowest level where transactions are isolated just enough to ensure that physically corrupt data is not read)
  • Read committed
  • Repeatable read
  • Serializable (the highest level, where transactions are completely isolated from one another)

SQLAPI++ maps different isolation levels on PostgreSQL server in the following way:

      SA_ReadUncommitted = 'READ COMMITTED'
      SA_ReadCommitted = 'READ COMMITTED'
      SA_RepeatableRead = 'READ COMMITTED'
      SA_Serializable = 'SERIALIZABLE'

For more details see SAConnection::setIsolationLevel.

 

Working with Long or Lob(CLob, BLob) data

SQLAPI++ supports four types for working with Long or Lob(CLob, BLob) data:

Name C enum constant
LongBinary SA_dtLongBinary
LongChar SA_dtLongChar
BLob (Binary Large object) SA_dtBLob
CLob (Character Large object) SA_dtCLob

The table below shows how SQLAPI++ data types correspond with servers original data types:

SA_dtLongBinary <= > BYTEA
SA_dtLongChar <= > TEXT
SA_dtBLob < = > Large Object
SA_dtCLob < => Large Object

Working with PostgreSQL Large Objects data (Oid field type) in PostgreSQL server has some particular features. Generally when you fetch Oid field data (which can point to any object, not necessary PostgreSQL Large Object) SQLAPI++ returns its value as a number (SQLAPI++ returns an object identifier). If you want to retrieve Oid field as PostgreSQL Large Object you should set command-related option OidTypeInterpretation to "LargeObject"value before the command execution. See SACommand::setOption for more detailes.

For more details see How To - Working with Long or Lob(CLob, BLob) data

 

Returning output parameters

In PostgreSQL output parameters are available immediately after calling SACommand::Execute.

SQLAPI++ Library automatically creates SAParam object to represent function return value. You can refer to this SAParam object using SQLAPI++ predefined name "RETURN_VALUE". When PostgreSQL function has output parameters SQLAPI++ "RETURN_VALUE" parameter is not assigned.

For more details see SACommand::Execute, SAParam object, How To - Returning Output Parameters.


Cancelling queries

Using SACommand::Cancel method you can cancel the following types of processing on a statement:

  • A function running asynchronously on the statement.
  • A function running on the statement on another thread.

SQLAPI++ calls PQrequestCancel function to cancel a query. To get more details see PQrequestCancel function description in PostgreSQL documentation.

For more details see SACommand::Cancel.

 

Connection, command, parameter and field options

A server specific option can relate to a connection, command,  parameter or field. We recommend you specify each option in an appropriate object, although it is possible to specify them in the parental object as well. In that case the option affects all the child objects.

A connection related option must be specified in a  SAConnection object.

A command related option may be specified in either SAConnection object or SACommand object. If it is specified in SAConnection object it affects all the commands on that connection.

A parameter related option may be specified in SAConnection object, SACommand object or SAParam object. If it is specified in SAConnection object it affects all the commands and therefore all the parameters on that connection. If it is specified in SACommand object it affects all the parameters on that command.

A field related option may be specified in SAConnection object, SACommand object or SAField object. If it is specified in SAConnection object it affects all the commands and therefore all the fields on that connection. If it is specified in SACommand object it affects all the fields on that command.

Specific options for PostgreSQL server:

Option name / Scope 

Description

LIBPQ.LIBS

Connection related. Should be specified before the first connection is made.

Forces SQLAPI++ Library to use specified LIBPQ library.

Valid values : Any valid LIBPQ library name list. Names separated by ';' on Windows or ':' on other OS.
Default value: Windows - "libpq.dll;pq.dll", Linux -  "libpq.so:libpq.so.5:libpq.so.4:libpq.so.3".

APPNAME

Connection related. Should be specified before connection is made.

Specifies the client application name (PostgreSQL 9.0 and higher 'application_name' configuration parameter).

Valid values: client application name string.
Default value: none.

OidTypeInterpretation

Command related 

Forces SQLAPI++ Library to interpret fields of Oid type as an object identifier or a Large Object.

Valid values: "LargeObject" and "Oid".
Default value: "Oid".

After you set "LargeObject" value to this option, SQLAPI++ interprets Oid type fields data as BLob data. Otherwise, it reads them as object identifiers (as ordinary numbers).

ClientEncoding

Connection related.

Uses PQsetClientEncoding to set up PostgreSQL client encoding.

Valid values:  See PostgreSQL documentation.

UseCursor

Command related.

Forces SQLAPI++ to use server side cursor within "select .." statement.

Valid values
: "True" and "1".  Default value: "False".

PreFetchRows

Command related.

Forces SQLAPI++ Library to fetch rows in bulk within the server side cursor, rather than retrieving records one by one.

Valid values: String containing number of rows in the fetch buffer.
Default value: "1".

UseDynamicCursor
or
Scrollable

Command related.
Forces SQLAPI++ to use scrollable dynamic server side cursor.

Valid values: "True", "1". Default value: "false".

For more details see SAConnection::setOption, SACommand::setOption, SAField::setOption, SAParam::setOption.

 

Getting native PostgreSQL API

You can call client specific API functions which are not directly supported by SQLAPI++ Library. SAConnection::NativeAPI method returns a pointer to the set of native API functions available for PostgreSQL. To use the database API directly you have to downcast this saAPI pointer to the appropriate type and use its implementation-specific members. The following table shows what type cast you have to make and what additional header file you have to include to work with PostgreSQL API.

Type casting

Additional
include file

Cast the result to class pgAPI:

saAPI *pResult = con.NativeAPI();
pgAPI *p_pgAPI = (pgAPI *)pResult;

#include <pgAPI.h>

To get more information about DBMS API functions see this DBMS specific documentation. 

For more details see SAConnection::NativeAPI.

 

Getting native PostgreSQL connection related handles

You have to use native API handles when you want to call specific PostgreSQL API functions which are not directly supported by the Library. API functions usually need to receive one or more active handles as a parameter(s). SAConnection::NativeHandles method returns a pointer to the set of native API connection related handles. To use API handles directly you have to downcast saConnectionHandles pointer to the appropriate type and use its implementation-specific members. The following table shows what type cast you have to make and what additional header file you have to include to work with specific PostgreSQL API.

Type casting

Cast the result to class pgConnectionHandles:

#include <pgAPI.h>

saConnectionHandles *pResult = con.NativeHandles();
pgConnectionHandles *p_pgCH =
                                (pgConnectionHandles *)pResult;

Available handles:

  • PGconn *conn;

To get more information about DBMS API functions and handles see this DBMS specific documentation. 

For more details see SAConnection::NativeHandles.

 

Getting native PostgreSQL command related handles

You have to use native API handles when you want to call specific PostgreSQL API functions which are not directly supported by the Library. API functions usually need to receive one or more active handles as a parameter(s). SACommand::NativeHandles method returns a pointer to the set of native API command related handles. To use API handles directly you have to downcast saCommandHandles pointer to the appropriate type and use its implementation-specific members. The following table shows what type cast you have to make and what additional header file you have to include to work with specific PostgreSQL API.

Type casting

Cast the result to class pgCommandHandles:

#include <pgAPI.h>

saCommandHandles *pResult = cmd.NativeHandles();
pgCommandHandles *p_pgCH =
                                (pgCommandHandles *)pResult;

Available handles:

  • PGresult *res;

To get more information about DBMS API functions and handles see this DBMS specific documentation. 

For more details see SACommand::NativeHandles.

 

Error handling

When an error occurs inside SQLAPI++ Library it throws an exception of type SAException. SAException::ErrPos method gets an error position in SQL statement. In PostgreSQL server SAException::ErrPos method returns -1 because PostgreSQL does not support this function.

For more details see How To - Error handling, SAException object.

 

Special header files - Compile time

The header files are in the include subdirectory of SQLAPI++ distributions:
#include <SQLAPI.h> - main header, should be used whenever SQLAPI++ is used.
#include <pgAPI.h> - PostgreSQL , should be included if direct libpq calls are required.

For more details see Online Documentation - Instructions for Compiling and Linking Applications with SQLAPI++ 

 

Problems and Questions

If you haven't found the answer to you questions or have some problems on using the Library, please, send e-mail to howto@sqlapi.com.