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Reference : Designing ODBC Applications for Performance Optimization : Retrieving Data : Reducing the Size of Data Retrieved

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Reducing the Size of Data Retrieved
Sometimes, long data must be retrieved. When this is the case, remember that most users do not want to see 100 KB, or more, of text on the screen.
To reduce network traffic and improve performance, you can reduce the size of data being retrieved to some manageable limit by calling SQLSetStmtAttr with the SQL_ATTR_MAX_LENGTH option.
Eliminating SQL_LONGVARCHAR, SQL_WLONGVARCHAR, and SQL_LONGVARBINARY data from the result set is ideal for optimizing performance.
Many application developers mistakenly assume that if they call SQLGetData with a container of size x that the ODBC driver only retrieves x bytes of information from the server. Because SQLGetData can be called multiple times for any one column, most drivers optimize their network use by retrieving long data in large chunks and then returning it to the user when requested. For example:
char CaseContainer[1000];
rc = SQLExecDirect (hstmt, "SELECT CaseHistory FROM Cases WHERE CaseNo = 71164", SQL_NTS);
rc = SQLFetch (hstmt);
rc = SQLGetData (hstmt, 1, CaseContainer,(SWORD) sizeof(CaseContainer), ...);
At this point, it is more likely that an ODBC driver will retrieve 64 KB of information from the server instead of 1 KB. In terms of network access, one 64-KB retrieval is less expensive than 64 retrievals of 1 KB. Unfortunately, the application may not call SQLGetData again; therefore, the first and only retrieval of CaseHistory would be slowed by the fact that 64 KB of data must be sent across the network.
Many ODBC drivers allow you to limit the amount of data retrieved across the network by supporting the SQL_MAX_LENGTH attribute. This attribute allows the driver to communicate to the database server that only x bytes of data are relevant to the client. The server responds by sending only the first x bytes of data for all result columns. This optimization substantially reduces network traffic and improves client performance. The previous example returned only one row, but consider the case where 100 rows are returned in the result set—the performance improvement would be substantial.