OpenEdge Data Management: DataServer for Microsoft SQL Server

Using the block cursor switches
ODBC options
The DataServer provides one switch to enable or disable block cursors, and two
switches to regulate memory consumption by block cursors. Block cursors also interact
with the query tuning cache size allocations.
Block cursors—the -Dsrv PRGRS_BLOCK_CURS,1 option
Block cursors are ON (1) by default. To disable block cursors for the entire connection,
set this option to OFF (0). The connection-level setting can be overridden for a specific
query with the query-tuning option: QUERY-TUNING(NO-LOOKAHEAD).
Block cursor allocation by table—the -Dsrv PRGRS_TABLE_BLOCKSIZE,nnn
option
The -Dsrv PRGRS_TABLE_BLOCKSIZE,nnn option where nnn is the size in bytes,
indicates the total allowable memory available to cursors of a given table. This value
puts a cap on the amount of memory all the open cursors against a given table can
consume. It does not determine an amount to be allocated. This upper limit on memory
allocation ensures that memory is not overutilized.
ODBC_DEF_TABBLOCKSZ is the default value for PRGRS_TABLE_BLOCKSIZE. It is currently
set at 65,000 bytes. At this rate approximately 18 tables open and utilizing all available
cursor space for block cusrors would utilize just 1 MB of memory. If record sizes are
large or the typical row count of your result sets is large and you have plenty of avilable
memory and your open table count is not too large, you might consider increasing this
size. If you are using served OpenEdge clients, keep in mind that the memory
requirments of the DataServer server executable on the server machine are
compounded by the number of users attached to the database through the same
broker.
Total Block cursor allocation—the -Dsrv PRGRS_MAX_BLOCKSIZE,nnn option
The -Dsrv PRGRS_MAX_BLOCKSIZE,nnn option where nnn is the size in bytes, indicates
the total allowable memory available to all cursors established as block cursors. This
value limits the total memory available to all block cursors irrespective of the table to
which it is allocated. ODBC_DEF_MAXBLOCKSZ sets the default size which is currently
1MB. The overall memory allocated to block cursors by the DataServer connection
cannot exceed the value set for PRGRS_MAX_BLOCKSIZE. Any block cursor that will place
memory consumption over this threshold will be rejected as a block cursor and instead
be established as a lookahead cursor.
Impact of block cursors on cache size—the -Dsrv QT_CACHE_SIZE,nnn option
The -Dsrv QT_CACHE_SIZE,nnn where nnn is the size in bytes of the cache to be
allocated for a cursor’s result set. This is a connection level default that can be
overridden at the query level. If the connection level cache size is not overridden at the
query level, then the query cache size times the number of open cursors cannot exceed
the maximum block areas for the table set by the -Dsrv PRGRS_TABLE_BLOCKSIZE
switch at any given time. The accumulation of query cache sizes from each block
cursor that has been allocated cannot exceed the total maximum block areas available
to block cursors as set by the -Dsrv PRGRS_MAX_BLOCKSIZE switch. If either block
cursor limit is reached, cursors will downgrade to lookahead cursoring. There is no
minimum for this value, however if two or more records cannot be returned to the
established block, a lookahead cursor is used. The query tuning cache size should be
set higher than the maximum record size times two to prevent cursor downgrades.
OpenEdge Data Management: DataServer for Microsoft SQL Server 331