Dick Kniep <dick@kniep.nl> writes:
> Does this mean that if you have a table that has many rows, and 95% of the
> rows contain a NULL value for a field, that indexing will be useless, because
> it will always do a tablescan?
Any time you have 95% of the rows of the table with the same value you're
probably going to have to think harder than usual about indexing strategies.
You might consider constructing your index with "WHERE a IS NOT NULL" since
otherwise 95% of your index is going to be full of records that aren't
selective enough to be useful anyways.
If you access your table with "WHERE a = ?" then it will undoubtedly use the
index effectively. You're accessing less than 5% (presumably much less), so
the index is useful.
--
greg
