add support to create indexes on non key fields
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Key: DERBY-3748
URL: https://issues.apache.org/jira/browse/DERBY-3748
Project: Derby
Issue Type: Improvement
Components: SQL, Store
Affects Versions: 10.4.1.3
Reporter: Mike Matrigali
Priority: Minor
Add support to CREATE INDEX on non key fields.
Often query execution can be improved by creating indexes that include all the
columns that are returned by the query (referred to as a "covering" index).
Currently in Derby to do this one must make all the fields part of the key, but
often the requirements of the query only need part of these columns to
be keyed. By making them part of the key it increases the size of the key that
is used in the non-key part of the index structure resulting in more
search overhead (both cpu and disk space), than is necessary.
The existing btree code has some framework already for non-key fields so a
major rework should not be necessary, but would not be surprised if some
issues still remain. Things to resolve as part of this work:
1) what part of sql to change, maybe just CREATE INDEX.
2) are there any optimizer/execution issues with the new type of index? any
metadata changes necessary to track key vs. non-key fields?
3) current btree code assumes non-key fields are at end, and use non-key field
stuff for unique vs. non-unique indexes.
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