OpenEdge Data Management: DataServer for Microsoft SQL Server

Indexes and sorting
Database design issues
You create and maintain all indexes from within the MS SQL Server data source using
native tools, rather than with the 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.
Give careful consideration to benefit and cost of creating indexes. Having indexes for
frequently executed queries can greatly improve record retrieval performance. An
abundance of unused or infrequently used indexes can have a negative impact on
performance due to the overhead cost of maintaining the indexes.
Using index definitions in the MS SQL Server data source, the DataServer builds index
information in the schema holder. OpenEdge index definitions for the data source
schema serve two purposes:
1. They allow you to use the OF option in ABL 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 custnum field is common to both the order and
customer tables, you could write the following statement:
FOR EACH order OF customer:
2. They translate USE–INDEX to SQL ORDER BY for DataServer operations. A MS SQL
Server data source uses the ORDER BY clause to assist in selecting the optimal
index for the query. For example, if you define city-dept as a MS SQL Server
data source primary key on the city and department fields, it is a unique index
in the schema holder. In this case, the following OpenEdge statements are
equivalent when accessing the data source:
FOR EACH employee USE-INDEX city-dept:
FOR EACH employee BY city BY department:
Note: If you do not specify a USE-INDEX or BY clause, your query will return
records in an unpredictable order. If your application requires a predictable
order, use include a USE-INDEX or BY clause.
OpenEdge Data Management: DataServer for Microsoft SQL Server 65