A bitmap index is a 2-dimensional table of bits.
when a certain value is OK, the bit is 1, otherwise it's 0

eg, table demo ( switchA char(1) )

values

A
B
A
C
D
E

the following table is created
--1 2 3 4 5 6" -> ID. There's a function that transforms this value to the ROWID if the row is selected
A 1 0 1 0 0 0
B 0 1 0 0 0 0
C 0 0 0 1 0 0
D 0 0 0 0 1 0
E 0 0 0 0 0 1
|--> possible values

When you do select ... where switchA = 'A'
--> all ID's where the 'A'-bit = 1 are selected, The ID is converted to a ROWID using a function. Now we have the rowid's off the records.


Hope this helps
Gert