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From "Todd Lipcon (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-6014) Support for block-granularity bitmap indexes
Date Thu, 31 May 2012 22:27:22 GMT

    [ https://issues.apache.org/jira/browse/HBASE-6014?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13287006#comment-13287006
] 

Todd Lipcon commented on HBASE-6014:
------------------------------------

I had some airplane time last week so I spent some of it thinking about how to go about this.
It's not quite as straight-forward in HBase as it is in your standard RDBMS -- at least on
the read path:

This is because a given row may actually span multiple "layers" of the stack of storefiles.
For example, for a query "col_a = 1 AND col_b = 1", we may have the following files in a region:

HFile 1: row1: col_a="1", no value for col_b
HFile 2: row1: col_a="2"  col_b="1"
(so the "current" version of the row has col_a=1 and col_b=1

So, if we naively apply the bitmap index to each HFile in turn, we would end up excluding
the block in both, and not see the correct result.

Instead, I think we have to apply each predicate to each HFile in turn to come up with a set
of ranges:

Predicate col_a = 1, HFile 1: output key range for block containing row1
Predicate col_a = 1, HFile 2: no output

Predicate col_b = 1, HFile 1: no output
Predicate col_b = 1, Hfile 2: output key range for block containing row1

For each predicate, we take the union of the key ranges across HFiles. Then, we take the intersection
across predicates, to come up with a total set of applicable key ranges. Then, we can push
this key range list down into the scanner stack to provide skip-ahead hints in the filter.

Any interns out there interested in this? :)
                
> Support for block-granularity bitmap indexes
> --------------------------------------------
>
>                 Key: HBASE-6014
>                 URL: https://issues.apache.org/jira/browse/HBASE-6014
>             Project: HBase
>          Issue Type: New Feature
>          Components: regionserver
>            Reporter: Todd Lipcon
>
> This came up in a discussion with Kannan today, so I promised to write something brief
on JIRA -- this was suggested as a potential summer intern project. The idea is as follows:
> We have several customers who periodically run full table scan MR jobs against large
HBase tables while applying fairly restrictive predicates. The predicates are often reasonably
simple boolean expressions across known columns, and those columns often are enum-typed or
otherwise have a fairly restricted range of values. For example, a real time process may mark
rows as dirty, and a background MR job may scan for dirty rows in order to perform further
processing like rebuilding inverted indexes.
> One way to speed up this type of query is to add bitmap indexes. In the context of HBase,
I would envision this as a new type of metadata block included in the HFile which has a series
of tuples: (qualifier, value range, compressed bitmap). A 1 bit in the bitmap indicates that
the corresponding HFile block has at least one cell for which a column with the given qualifier
falls within the given range. Queries which have an equality or comparison predicate against
an indexed qualifier can then use the bitmap index to seek directly to those blocks which
may contain relevant data.

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