Daniel Bausch wrote:
> Hello Jonah, Simon, and the hackers,
>
> I am going to implement a simple kind of "encoded bitmap indexes"
(EBI).
> That is an index type where the bitmap columns may not only contain
> only a single '1' in the set of bits belonging to a tuple. Instead,
an
> additional mapping table translates the distinct values of the table
> column into a unique encoding. To select for a given value all bitmap
> columns must be compared instead of only one. Queries that match
> multiple different values (like IN lists or range queries) simplify to
> less than the full set of bitmaps that needs to be compared because of
> boolean logic. The total number of bitmaps required to represent
unique
> encodings for all different values is ceil(ld(n)), where n is the
number
> of distinct values. Compared to normal bitmap indexes this solves the
> problem of high-cardinality columns. It is targetet at data
warehousing
> scenarios with insert only data.
>
> The respective scientific paper can be found at
> http://www.dvs.tu-darmstadt.de/publications/pdf/ebi_a4.pdf
I cannot answer your questions, but I read the paper and have some
questions myself.
1) As you mention, a WHERE clause that checks for only one value will be more expensive with an encoded bitmap index
thanwith a regular bitmap index. If you want to implement encoded bitmap indexes, wouldn't it be good to also
implementregular bitmap indexes so that the user has a choice?
2) The paper mentions that finding a good encoding and simplifying bitmap access for a certain query are nontrivial
problems. Moreover, an encoding is good or bad only with respect to certain queries, which the system does not know at
index creation time. Do you have any ideas how to approach that? If not, the paper suggests that, with enough values
tocheck for, even a non-optimized encoded bitmap index should perform much better than a normal bitmap index, so
maybethat's the way to go (maybe only encode the NULL value as all zeros).
Yours,
Laurenz Albe