Hi,
I have a puzzle.
A bitmap index on a varchar2(25) column. table has 7131413 rows, of which
7125290 are null for the column in question, the rest of the rows are unique
values. There is a histogram on this column, it has 2 buckets.
A select statement using this column in the where clause generates two
plans, one which uses the index and one which doesn't. The deciding factor
is the length of the string in the filter.
select * from table where microchip_number = 'avid1'
this produces a plan which uses the bitmap
select * from table where microchip_number = 'avid12' (and any additional
characters in the string )
this produces a plan which uses a full table scan.
I am stumped about how to figure out why the optimizer makes a choice based
upon the length of the string in the predicate.
Much obliged,
Josh
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Author: Josh Collier
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