Re: Oracle Odbc Driver Configuration Fetch Buffer Size
Melva Simons
Open Database Connectivity (ODBC) provides a standard interface that allows one application to access many different data sources. The application's source code does not have to be recompiled for each data source. A database driver links the application to a specific data source. A database driver is a dynamic-link library that an application can invoke on demand to gain access to a particular data source. Therefore, the application can access any data source for which a database driver exists.
The following figure shows the components of the ODBC model. The model begins with an ODBC application making a call to the Driver Manager through the ODBC application program interface (API). The Driver Manager can be either the Microsoft Driver Manager or the unixODBC Driver Manager. Still using the ODBC API, the Driver Manager makes a call to the ODBC Driver. The ODBC Driver accesses the database over a network communications link using the database API. This figure shows an ODBC application accessing three separate databases.
The Oracle ODBC Driver enables ODBC applications on Microsoft Windows, as well as UNIX platforms like Linux, Solaris, and IBM Advanced Interactive eXecutive (AIX) read and write access to Oracle databases through the ODBC interface using Oracle Net Services software.
The Oracle ODBC Driver uses the Oracle Call Interface (OCI) client and server software to submit requests to and receive responses from the data source. Oracle Net Services communications protocol is used for communications between the OCI client and the Oracle server.
The Oracle ODBC Driver translates ODBC SQL syntax into syntax that can be used to access the data source. When the results are returned from the data source, the Oracle ODBC Driver translates them back to ODBC SQL syntax.
* The Oracle ODBC Resource data definition language (DLL) file (sqresxx.dll), where xx represents the language abbreviation, contains all pertinent language information; the default resource file used is sqresus.dll.
The Oracle ODBC Driver is broadly compatible with the SQL-99 Core specification which is a superset of the SQL-92 Entry Level specification. Applications must call SQLGetInfo with the appropriate information type to retrieve a list of SQL-99 supported features.
Before Configuring the Data Source, you must configure network database services so there is an entry for each Transparent Network Substrate (TNS) Service Name. To do this, use Oracle Net Configuration Assistant (NETCA).
Before an application can communicate with the data source, you must provide configuration information. The configuration information informs the Oracle ODBC Driver as to which information you want to access.
The data source consists of the data that you want to access, its associated operating system, database management system, and network platform used to access the database management system. The data source for requests submitted by the Oracle ODBC Driver is an Oracle database and supports transports available under Oracle Net Services.
After you have installed the Oracle ODBC Driver, use the ODBC Data Source Administrator to configure or add an Oracle data source for each of your Oracle databases. The Oracle ODBC Driver uses the information you enter when you add the data source to access the data. Follow these steps:
Click Finish. The Oracle ODBC Driver Configuration Dialog Box is displayed. You must enter the DSN and TNS Service Name. You can provide the other information requested in the dialog box, or you can leave the fields blank and provide the information when you run the application.
The following list is an explanation of the main setup options and fields found on the Oracle ODBC Driver Configuration dialog box shown in the preceding graphic. The tabs found on the lower half of this dialog box are described in subsequent topics.
TNS Service Name - The location of the Oracle database from which the ODBC driver will retrieve data. This is the same name entered in Configuring Oracle Net Services using the Oracle Net Configuration Assistant (NETCA). For more information, see the NETCA documentation and About Using the Oracle ODBC Driver for the First Time. The TNS Service Name can be selected from a pull-down list of available TNS names. For example, "ODBC-PC". You must enter a TNS Service Name.
You must enter the DSN and the TNS Service Name. You can provide the other information requested in the dialog box or you can leave the fields blank and provide the information when you run the application.
In addition to the main setup options previously described, there is a Test Connection button available. The Test Connection button verifies whether the ODBC environment is configured properly by connecting to the database specified by the DSN definition. When you press the Test Connection button, you are prompted for the username and password.
Enable Result Sets - Enables the processing of Oracle Result Sets. If Result Sets are not required for your application, Result Set support can be disabled. There is a small performance penalty for procedures called from packages not containing Result Sets. Result Sets are enabled by default.
Enable Query Timeout - Enables query timeout for SQL queries. By default, the Oracle ODBC Driver supports the SQL_ATTR_QUERY_TIMEOUT attribute for the SQLSetStmtAttr function. If this box is not checked, the Oracle ODBC Driver responds with a "not capable" message. Query Timeout is enabled by default.
Enable Closing Cursors - Enables closing cursors. By default, closing cursors is disabled (the field is empty), meaning a call to close a cursor does not force the closing of OCI cursors when this behavior is not desired because it can cause an unnecessary performance hit. Enable closing cursors when you want to force the closing of OCI cursors upon a call to close a cursor.
Enable Thread Safety - Thread safety can be disabled for a data source. If thread safety is not required, disabling this option eliminates the overhead of using thread safety. By default, thread safety is enabled.
Numeric Settings - Allows you to choose the numeric settings that determine the decimal and group separator characters when receiving and returning numeric data that is bound as strings. This option allows you to choose Oracle NLS settings (the default setting), Microsoft default regional settings (to provide a way to mirror the Oracle OLE DB driver's behavior for greater interoperability), or US numeric settings (which are necessary when using MS Access or DAO (Database Access Objects) in non-US environments).
Fetch Buffer Size - The amount of memory used to determine how many rows of data the ODBC Driver prefetches at a time from an Oracle database regardless of the number of rows the application program requests in a single query. However, the number of prefetched rows depends on the width and number of columns specified in a single query. Applications that typically fetch fewer than 20 rows of data at a time improve their response time, particularly over slow network connections or to heavily loaded servers. Setting Fetch Buffer Size too high can make response time worse or consume large amounts of memory.
When LONG and LOB data types are present, the number of rows prefetched by the ODBC Driver is not determined by the Fetch Buffer Size. The inclusion of the LONG and LOB data types minimizes the performance improvement and could result in excessive memory use. The ODBC Driver disregards Fetch Buffer Size and prefetches a set number of rows only in the presence of the LONG and LOB data types.
Enable LOBs - Enables the writing of Oracle LOBs. If writing Oracle LOBs is not required for your application, LOB support can be disabled. There is a small performance penalty for insert and update statements when LOBs are enabled. LOB writing is enabled by default but disabled for Oracle databases that do not support the LOB data type.
Enable Statement Caching - Enables statement caching feature, which increases the performance of parsing the query, in case the user has to parse the same text of query and related parameters multiple times. The default is disabled.
Cache Buffer Size - The statement cache has a maximum size (number of statements) that can be modified by an attribute on the service context, OCI_ATTR_STMTCACHESIZE. The default cache buffer size is 20 that are used only if statement caching option is enabled. Setting cache buffer size to 0 disables statement caching feature.
Translate ORA errors - Any migrated third party ODBC application, which is using the SQL Translation Framework feature, expects that errors returned by the server to be in their native database format, then users can enable this option to receive native errors based on the error translation registered with SQL Translation Profile.
Enable Failover - Enables Oracle Fail Safe and Oracle Parallel Server failover retry. This option in an enhancement to the failover capabilities of Oracle Fail Safe and Oracle Parallel Server. Enable this option to configure additional failover retries. The default is enabled.
Force SQL_WCHAR Support - Check this box to enable SQLDescribeCol, SQLColumns, and SQLProcedureColumns to unconditionally return the data type of SQL_WCHAR for SQL_CHAR columns; SQL_WVARCHAR for SQL_VARCHAR columns; and SQL_WLONGVARCHAR for SQL_LONGVARCHAR columns. This feature enables Unicode support in applications that rely on the results of these ODBC calls (for example, ADO). This support is disabled by default.
Set Metadata Id Default to SQL_TRUE - Check this box to change the default value of the SQL_ATTR_METADATA_ID connection and statement attribute at connection time to SQL_TRUE. Under normal circumstances, SQL_ATTR_METADATA_ID would default to SQL_FALSE. ODBC calls made by the application to specifically change the value of the attribute after connection time are unaffected by this option and complete their functions as expected. By default, this option is off.
Prefetch size for LONG column data - Set this value to prefetch LONG or LONG RAW data to improve performance of ODBC applications. This enhancement improves the performance of Oracle ODBC driver up to 10 times, depending on the prefetch size set by the user. The default value is 0. The maximum value that you can set is 64 KB (65536 bytes).
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