OpenEdge Data Management: DataServer for Microsoft SQL Server

Database design issues
restrictions. While the server's CLOB-style definition may contain character data that exceeds
30,000 bytes, only approximately 30,000 bytes of that data can be retrieved and manipulated by
the CHARACTER data type in ABL client. If the server data associated with a CLOB-style server
definition is represented by UCS-2 character data in double-byte format, then the 30,000 /byte/
restriction on the CHAR data type also becomes an approximately 15,000 /character /restriction
in the Uniocde-enabled ABL client.
Indexes and sorting
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.
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