Block cursors are the default over look-ahead cursors when PRGRS_BLOCK_CURS is enabled and a server-side cursor is needed. The switch -Dsrv PRGRS_BLOCK_CURS,0 will turn off block cursors on ODBC data sources. You can always disable block cursors for a specific query by disabling lookahead with a query tuning option: QUERY-TUNING(NO-LOOKAHEAD). Block cursors is not a one-size-fits-all feature. There are application environments where the default values for block allocation sizes can result in either under-utilization or over-utilization of memory. See OpenEdge Data Management: DataServer for Microsoft SQL Server for information on tuning the amount of memory that block cursors utilizes.
For any given query, the use of a block cursor or a look-ahead cursor is mutually exclusive. In certain circumstances, block cursors will downgrade to lookahead. These are the criteria:
When -Dsrv BINDING,0 is set. When binding is off, block cursors are disabled.
Predictable single record result sets—FOR FIRST/FOR LAST or dynamic find operations.
The PRGRS_TABLE_BLOCKSIZE has been exceeded and all existing block table space is in use by open cursors.
The accumulation of memory allocated to all block cursors has reached PRGRS_MAX_BLOCKSIZE.
The QT_CACHE_SIZE query-tuning (CACHE-SIZE) value is not larger than two times the maximum row size. At least two rows need to fit in the result block to use a block cursor.
Note: The macro BLOCKBUF_AVAILABLE can be used to determine if the current query is or is not using a block cursor.
When -Dsrv BINDING,0 is set. When binding is off, block cursors are disabled.