ACC: How to Optimize MS Access Using ODBC Data Sources

• Microsoft Access versions 2.0, 7.0, 97

SUMMARY

Advanced: Requires expert coding, interoperability, and multiuser skills.

This article describes several tips for improving performance when you are accessing data from an ODBC data source.

MORE INFORMATION

Try these tips to improve performance with ODBC data sources:

• Restrict the amount of data that you request from the server. Do not ask for more data than you need. Use queries to select only the fields and rows that you need.
• Use only the functionality you need. Snapshots are less powerful than dynasets and are not updateable, but may be faster, particularly for small recordsets without Memo or OLE Object fields.
• Use attached tables to access server data. Do not use "direct" server access (that is, opening remote databases and executing queries against them). Instead, attach tables and use the attachments, or use pass- through queries.
• Use list and combo boxes wisely. On a form, each list box, combo box, subform, and control containing a total requires a separate query. Against local data, performance may be adequate. Against remote data, however, long delays may occur when you open a form since each query must be sent to the server and a response returned before the form can open.
• Avoid large combo boxes. Including a combo box with hundreds, or even thousands, of choices based on a local table may yield acceptable response time, especially if you define an appropriate index on the local table. Against a remote table, however, such a combo box will yield sluggish performance as it drains server and network resources as it fetches data to fill the list. It is best to limit the number of rows returned to the combo box when you are working with remote data. You can also break up the data into smaller combo boxes (bearing the tip above in mind).
• Use the Find command only on smaller recordsets. The Microsoft Jet database engine optimizes the Find command to work well against local recordsets of almost any size, and against remote recordsets of reasonable size. However, when you are working with large remote recordsets (thousands of records or more), you should instead use a filter or query and be careful to use restrictions that your server can process.
• Make sure that queries are sent to the server for processing. The most important factor in query performance against remote data is ensuring that your server executes as much of the query as possible. The Microsoft Jet database engine attempts to send the entire query to your server, but evaluates locally any query clauses and expressions that are not generally supported by servers or by your particular server. Functionality not supported by servers in general includes:

      - Operations that cannot be expressed in a single SQL statement. This
        can occur when you use a query as an input to another query, or when
        your query's FROM clause contains a Totals query or DISTINCT query.
        Often, you can rearrange your queries to calculate totals after all
        other operations.
  

      - Operations that are Microsoft Jet database engine-specific extensions
        to SQL, such as crosstab queries, TOP queries, and reports with
        multiple levels of grouping and totals. Note that simple crosstab
        queries can be sent to servers.
  

      - Expressions that contain Microsoft Access-specific operators or
        functions. The Microsoft Access financial functions and statistical
        aggregates have no server equivalents.
  

      - User-defined Access Basic functions that take remote columns as
        arguments. These functions do not exist on the server, but must
        process remote column data. However, if a user-defined function
        returns a single value and does not reference a remote column, the
        function is evaluated locally and its value is sent to the server
        for processing.
  

      - Mixing text and numeric data types in operators or UNION query
        outputs. Most servers lack the data-type leniency in Microsoft
        Access. Because of this, use explicit conversion functions where
        appropriate.
  

      - Heterogeneous joins between local tables and remote tables, or
        between remote tables in different ODBC data sources. Joins between
        small local tables and large remote tables, where the join column is
        indexed, may result in a remote index join. In a remote index join,
        one query for each row in the local table is sent to the server and
        only the joining rows are returned.
  

      - Non-remotable expressions, or expressions that cannot be sent
        remotely, because they cannot be evaluated by your server. Non-
        remoteable output expressions (those in the SELECT clause) do not
        force local evaluation of your query unless they occur in a Totals
        query, a DISTINCT query, or a UNION query. Non-remoteable expressions
        in other clauses (WHERE, ORDER BY, GROUP BY, HAVING, and so on) force
        at least part of your query to be evaluated locally.
  

• Servers differ in some areas of supported functionality. When you attach a remote table, the Microsoft Jet database engine queries the ODBC driver for its capabilities. If the required functionality is supported by the driver and the server, the Microsoft Jet database engine sends the operation to the server for processing. If not, the Microsoft Jet database engine performs the operation locally. Areas of differing support include (but are not limited to) the following:

      - Outer joins. Note that the Microsoft Jet database engine does not
        send multiple outer joins to a server, although many inner joins may
        accompany a single outer join.
  

      - Numeric, string, and date/time functions (such as Log(), Mid$(),
        DatePart(), and so on).
  

      - Conversion functions (such as CInt(), CStr(), CVDate(), and so on).