State of the on-disk bitmap index

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 than with
a regular bitmap index. If you want to implement encoded bitmap
indexes, wouldn't it be good to also implement regular 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 to check for,
even a non-optimized encoded bitmap index should perform
much better than a normal bitmap index, so maybe that's the way
to go (maybe only encode the NULL value as all zeros).

Yours,
Laurenz Albe

Am 20.08.2012 09:40, schrieb Albe Laurenz:

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 than with
a regular bitmap index. If you want to implement encoded bitmap
indexes, wouldn't it be good to also implement regular bitmap
indexes so that the user has a choice?

Sorry if that one was not clear: The first thing, I am going to do, is
to work on the normal bitmap indexes (the one based on the Bizgres
patch). I want to port it to master HEAD and give it back to the
community. After that I want to base my EBI implementation on that.
Eventually, I will publish that implementation, too. (After doing
tuning, experiments, and make sure it works well.)

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.

Actually, I was not involved in writing that paper. I want to use that
idea to show something different. I know of a follow up work by Golam
Rabilul Alam et al. that uses the query history and data mining on that
to optimize for the most common cases. There may be others. A more
detailed discussion of EBI can also be found in:

http://www-old.dvs.informatik.tu-darmstadt.de/staff/wu/query.TR.ps.gz

Do you have any ideas how to approach that?
If not, the paper suggests that, with enough values to check for,
even a non-optimized encoded bitmap index should perform
much better than a normal bitmap index, so maybe that's the way
to go (maybe only encode the NULL value as all zeros).

Actually "all zeros" is reserved for "non-existent" (a.k.a. "deleted" or
"invisible").

The thing with the "enough values" is a bit problematic, indeed, because
even a DBA cannot influence how the queries of the user or the user
application look like. You will not use encoded bitmap indexes or
normal bitmap indexes for a column that is usually point accessed like
the ID column. For that you will stick to hash or tree indexes.

Kind regards,
Daniel

--
Daniel Bausch
Wissenschaftlicher Mitarbeiter
Technische Universität Darmstadt
Fachbereich Informatik
Fachgebiet Datenbanken und Verteilte Systeme

Hochschulstraße 10
64289 Darmstadt
Germany

Tel.: +49 6151 16 6706
Fax: +49 6151 16 6229

Am 20.08.2012 11:44, schrieb Daniel Bausch:

Actually, I was not involved in writing that paper. I want to use that
idea to show something different. I know of a follow up work by Golam
Rabilul Alam et al. that uses the query history and data mining on that
to optimize for the most common cases. There may be others. A more
detailed discussion of EBI can also be found in:

http://www-old.dvs.informatik.tu-darmstadt.de/staff/wu/query.TR.ps.gz

Oops, that was the wrong link. I meant this one:
http://www-old.dvs.informatik.tu-darmstadt.de/staff/wu/bitmap.ps.gz

--
Daniel Bausch
Wissenschaftlicher Mitarbeiter
Technische Universität Darmstadt
Fachbereich Informatik
Fachgebiet Datenbanken und Verteilte Systeme

Hochschulstraße 10
64289 Darmstadt
Germany

Tel.: +49 6151 16 6706
Fax: +49 6151 16 6229