Chapter 5: Access Structures

Chapter 5: Access Structures

Overview

Data access methods are not part of the SQL standard, so each vendor is free to do anything they want as a vendor extension. When the ANSI X3H2 Database Standards Committee (now called NCITS H2) decided to exclude index creation from the SQL-86 standard, the reason given was that indexing and accessing data was too physical for a standard that was trying to be as abstract as possible. There was also some debate in the committee that introducing a CREATE INDEX command would also require us to introduce a CREATE HASH command, a CREATE LINK command, and assorted CREATE commands for an endless list of access methods.

However, in the real world, it is vital to have some method to improve access to the data? simple sequential file access would be too slow to be practical. The X/Open Consortium came up with this syntax, based on the most common vendor syntax in use at the time, for handling indexing:

CREATE INDEX ON ();
DROP INDEX ;

This basic syntax can then have additional product-specific clauses and keywords that modify the index in some way. However, every product agrees that the user can only CREATE and DROP an index. The SQL engine handles any references to the access without requiring the user to code for them in SQL, though the product might provide ways of forcing the use of a particular access path.

The important questions concerning access methods in a...

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