Description
For those interfaces that support it, Block specifies the cursor blocking factor when connecting to a database. The blocking factor determines the number of rows that a DataWindow object can fetch from the database at one time.
Using the Block parameter can improve performance when accessing a database in PowerBuilder.
Applies to
ODBC (if driver and back-end DBMS support this feature)
OLE DB
O90 Oracle9i
O10 Oracle 10g
ORA Oracle (for 11g and later)
SNC SQL Native Client for Microsoft SQL Server
MSOLEDBSQL Microsoft OLE DB Driver for SQL Server
Syntax
Block=blocking_factor
|
Parameter |
Description |
|---|---|
|
blocking_factor |
The number of rows you want the DataWindow object to fetch from the database at one time. The blocking factor can be a number from 1 to 1000, inclusive. To turn off block fetching, set Block to 1. |
Default value
The default value for the Block parameter depends on the DBMS you are accessing, as summarized in the following table:
|
DBMS |
Block default value |
|---|---|
|
ODBC |
For most DataWindow objects, the Block default value is the following, up to a maximum of 32K per column: Block=1000 If you specified that the DataWindow object should retrieve only as many rows as needed from the database (Retrieve.AsNeeded property), the Block default value is the following, up to a maximum of 32K per column: Block=100 |
|
OLE DB |
PowerBuilder sets the blocking factor to 1 |
|
Oracle |
PowerBuilder sets the blocking factor dynamically if one row size is too large. By default the blocking factor is 300 rows. |
Using the default blocking factor
You should not have to set a non-default value for Block. In most cases, the default blocking factor used by PowerBuilder should meet your needs.
Usage
Requirements for ODBC data sources
To use the Block database parameter with an ODBC data source, your ODBC driver must:
-
Be ODBC Version 2.0 compliant or higher, and
-
Support the SQLExtendedFetch API call
The SQL Anywhere ODBC driver that comes with PowerBuilder meets both of these requirements.
For information about whether your ODBC driver meets these requirements, see the documentation that comes with your driver.
Determining the Block value for ODBC data sources
PowerBuilder searches the following in this order to determine the Block value for ODBC data sources:
-
The section for your database profile in the registry or the value of the Transaction object property (in an application)
-
The section for your ODBC driver in the PBODB initialization file
If PowerBuilder does not find a Block value in these locations, it uses the default Block value for the DBMS you are accessing.
Turning off block fetching
To turn off block fetching for an ODBC data source or Oracle database, set the Block parameter to 1.
OLE DB and Microsoft SQL Server
When you use the OLE DB database interface with a Microsoft SQL Server database and retrieve data into a DataWindow or use an embedded SQL cursor in a SELECT statement, server-side cursors are used to support multiple command execution. If this has a negative impact on performance, try increasing the size of the Block database parameter to 500 or more, or adding the following line to the [Microsoft SQL Server] section in the PBODB.ini file to turn off server-side cursors:
ServerCursor='NO'
Oracle and MaxFetchBuffer
For Oracle, the Block parameter can be used in conjunction with the MaxFetchBuffer database parameter to improve performance when the size of a row is very large. The MaxFetchBuffer parameter has a default value of 5000000 bytes, which is sufficient for most applications. The size of the actual fetch buffer is the product of the value of the blocking factor and the size of the row.
If the fetch buffer required by the blocking factor and the row size is greater than the value of MaxFetchBuffer, the value of the blocking factor is adjusted so that the buffer is not exceeded. For example, if block=500 and the row size is 10KB, the fetch buffer is 5000KB, which equals the default maximum buffer size.
Examples
To set the blocking factor for DataWindow objects to 50 rows:
-
Database profile
Type 50 in the Retrieve Blocking Factor box on the Transaction page in the Database Profile Setup dialog box:
-
Application
Type the following in code:
SQLCA.DBParm="Block=50"
See also
