Progress
DataServer for
Microsoft SQL Server
Guide
Indexes
You create and maintain all indexes from within the MSS data source, using native tools, rather than with the Progress Data Dictionary. A data source index uses a logical pointer to the physical locations of table rows in order to sequence data access. You can add and drop indexes but you cannot use their names in queries. The data source alone ultimately decides when and how to use indexes; its decisions are not affected by the DataServer.
Using index definitions in the MSS data source, the DataServer builds index information in the schema holder. Progress index definitions for the data source schema serve two purposes:
- They allow you to use the OF option in Progress with the FOR EACH and FIND statements. Using the OF option improves the readability of your code. The OF keyword is equivalent to the SQL WHERE clause. You can use OF only when you have a field of the same name in two tables and the field is an index in at least one of the tables. Therefore, since the cust-num field is common to both the order and customer tables, you could write the following statement:
- Progress translates USE–INDEX to SQL ORDER BY for DataServer operations. For example, if you define city-dept as an MSS data source primary key on the city and department fields, it is a unique index in the schema holder. In this case, the following Progress statements are equivalent when accessing the data source:
NOTE: If you do not specify a USE–INDEX or BY clause, your query will return records in an unpredictable order. Your application might not require predictable ordering, but if it does, be sure to include USE–INDEX or BY clause in your query definition.
Dummy Indexes for Sort Order
You can add dummy index definitions to your schema holder independent of the actual index definitions in your MSS data source to provide a sort order. These dummy indexes can improve the readability of your Progress 4GL code. An index in the schema holder need not match an index in the MSS data source. However, the absence of an index definition in the data source can impact performance when you retrieve data with selection on the dummy indexes.
Unique Indexes
If your MSS data source tables have at least one unique index, they can be used to support operations such as backward and forward scrolling and accurate cursor positioning through the FIND CURRENT, PREV, and LAST statements. If a table does not have a unique index, you can only scroll forward through its data.
If an MSS data source table does not have a unique index, you can designate an index to serve as the unique index for the schema holder. An index that you designate as unique in the schema holder must be unique with respect to the data in the data source, otherwise you receive run-time errors. See "The DataServer Tutorial," for instructions on using the Progress Data Dictionary to designate unique indexes.
MSS data source views and result sets from stored procedures do not have unique indexes. Just as for tables, you can use the Progress Data Dictionary to create a unique index in the schema holder based on fields in a view or result set so that you can browse data accessed through views or stored procedures.
NOTE: Do not change the designated ROWID key of a record while an application is running. Suppose, for example, that cust-num is a unique key and has been designated the Progress ROWID. If a user changes the value of cust-num for a customer from 1 to 111, other users receive an error message when they try to access the record for customer 1.
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