Microsoft KB Archive/209551

= ACC2000: Tips for Optimizing Queries on Attached SQL Tables =

Article ID: 209551

Article Last Modified on 7/16/2004

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APPLIES TO


 * Microsoft Access 2000 Standard Edition

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This article was previously published under Q209551



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

This article applies only to a Microsoft Access database (.mdb).



SUMMARY
Special considerations must be made for performance optimization of queries built on attached SQL database tables. An SQL database, for this article, is defined as any client/server database that supports some level of ANSI SQL as an intrinsic part of the database's programming language. The optimal query strategy for these queries is to ensure that all query operations are performed on the server. This article outlines tips on how to ensure that queries against attached SQL database tables are performed on the server.

This article assumes a basic understanding of client/server computing environments and architectures.



MORE INFORMATION
The key to improving query performance on attached SQL database tables is to ensure that no data is filtered on the client. Filtering data on the client increases network traffic and does not allow for leveraging of advanced server hardware, essentially turning a client/server system into a file server system. To this end, keeping the query evaluation processes on the server side reduces overhead and keeps an application running as fast as possible.

Generic query optimization techniques should not be ignored when you are using attached SQL database tables. Restrictions on WHERE clauses, such as equality or range comparisons and sorting, should still be performed on indexed fields. For more information on SQL Server 7.0 query optimization, see the following on the Microsoft TechNet Web site:

http://www.microsoft.com/technet/sql/inside14.asp

Other SQL database vendors may provide the same information for their customers.

Use care when implementing intrinsic or user-defined functions in query fields or when using criteria that are not supported on the server. Generally, SQL databases have functionality that corresponds to most standard Microsoft Access functions, but each server is different.

Many intrinsic Microsoft Access functions have direct back-end correspondents. Microsoft Access asks the ODBC driver about intrinsic function support and performs the appropriate mappings.

You can use user-defined functions and Microsoft Access intrinsic functions without server equivalents when they are accompanied by server-capable restrictions that restrict the data. For example, the following query

Query1a: SELECT * FROM MillionRowTable WHERE Funk1(col1) = 10

returns the whole table and evaluates "Funk1(col1) = 10" locally, whereas the following query

Query1b: SELECT * FROM MillionRowTable WHERE Funk1(col1) = 10

AND LastName BETWEEN 'g' AND 'h'

sends the "BETWEEN 'g' AND 'h'" restriction to the server, returns the qualifying rows, and evaluates "Funk1(col1)" on only those rows.

Non-remote-capable SELECT list items do not force a query to be executed locally, unless they are used with unique values or a totals query (DISTINCT/GROUP BY). For example, the following query

Query2a: SELECT * FROM MyTable WHERE Format(col1, ...) = 10

returns the whole table and causes the WHERE clause to be evaluated locally. However, the following query

Query2b: SELECT Format(col1,...) FROM MyTable WHERE col2 = 10

sends "SELECT col1 FROM MyTable WHERE col2 = 10" to the server, presumably returning far less data over the network. It then locally evaluates Format on the col1 values returned.

Of the following two queries, Query3a is sent completely to the server. Query3b sends "SELECT col1 FROM MyTable" and performs the Format function, and therefore the DISTINCT clause, locally.

Query3a: SELECT DISTINCT col1 FROM MyTable

Query3b: SELECT DISTINCT Format(col1,...) FROM MyTable

The following two queries are performed as follows: Query4a is sent completely to the server; Query4b sends "SELECT col1 FROM MyTable" and performs the StdDev aggregate function locally, because it is not a SQL standard function.

Query4a: SELECT Sum(col1) FROM MyTable

Query4b: SELECT StdDev(col1) FROM MyTable

Crosstab queries present unique restrictions, some pertinent to all queries, some to crosstab queries only:
 * 1) Only standard aggregate (totals) functions, such as Count, Sum, Min, Max, and Avg can be used.
 * 2) Aggregate functions cannot be used as row or column headers.
 * 3) Only one aggregate function can be used in the value.
 * 4) Nothing can be sorted.
 * 5) If a fixed-value list of column headers is supplied, the value cannot be embedded in an expression.

Restriction #1 applies to all queries against SQL databases from Microsoft Access. Except for #4, the others are uncommon and are crosstab/SQL database-specific. Crosstab queries are not SQL standard, and are not generally supported on SQL database servers. However, if the rules above are not violated (which is the case for most simple crosstab queries), Microsoft Access can reformulate the query as a standard GROUP BY query, send it remotely, return only the aggregation result, and transform it into a crosstab locally.

Open-ended restrictions do not use indexes on SQL databases. Typically, a SQL WHERE clause reading

WHERE col1 > 1000

is slower than

WHERE col1 between 1000 and 1000000000

This is a server problem, not a Microsoft Access problem, but it can affect Microsoft Access performance when an SQL database is used as a back end.

When you are using wildcard characters, take special care to ensure that you use the correct wildcards. SQL Server supports the use of % and _ rather than ? and * for wildcards.

Keywords: kbhowto kbinfo kbusage KB209551

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