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SQL Anywhere 11.0.1 (Français) » SQL Anywhere Server - Programming » Introduction to Programming with SQL Anywhere » Using SQL in applications » SQL Anywhere cursors » Cursor sensitivity and performance

 

Prefetching rows

Prefetches and multiple-row fetches are different. Prefetches can be carried out without explicit instructions from the client application. Prefetching retrieves rows from the server into a buffer on the client side, but does not make those rows available to the client application until the application fetches the appropriate row.

By default, the SQL Anywhere client library prefetches multiple rows whenever an application fetches a single row. The SQL Anywhere client library stores the additional rows in a buffer.

Prefetching assists performance by cutting down on client/server round trips, and increases throughput by making many rows available without a separate request to the server for each row or block of rows.

For more information about controlling prefetches, see prefetch option [database].

Controlling prefetching from an application
  • The prefetch option controls whether prefetching occurs. You can set the prefetch option to Always, Conditional, or Off for a single connection. By default, it is set to Conditional.

  • In embedded SQL, you can control prefetching on a per-cursor basis when you open a cursor on an individual FETCH operation using the BLOCK clause.

    The application can specify a maximum number of rows contained in a single fetch from the server by specifying the BLOCK clause. For example, if you are fetching and displaying 5 rows at a time, you could use BLOCK 5. Specifying BLOCK 0 fetches 1 record at a time and also causes a FETCH RELATIVE 0 to always fetch the row from the server again.

    Although you can also turn off prefetch by setting a connection parameter on the application, it is more efficient to specify BLOCK 0 than to set the prefetch option to Off. See prefetch option [database].

  • Prefetch is disabled by default for value sensitive cursor types.

  • In Open Client, you can control prefetching behavior using ct_cursor with CS_CURSOR_ROWS after the cursor is declared, but before it is opened.

Prefetch dynamically increases the number of prefetch rows in cases that are likely to result in improved performance. This includes cursors that meet the following conditions:

  • They use one of the supported cursor types:

    • ODBC and OLE DB   FORWARD-ONLY and READ-ONLY (default) cursors

    • Embedded SQL   DYNAMIC SCROLL (default), NO SCROLL, and INSENSITIVE cursors

    • ADO.NET   all cursors

  • They perform only FETCH NEXT operations (no absolute, relative, or backward fetching).

  • The application does not change the host variable type between fetches and does not use a GET DATA statement to get column data in chunks (using one GET DATA statement to get the value is supported).