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From "stack (JIRA)" <j...@apache.org>
Subject [jira] Created: (HBASE-2037) Alternate indexed hbase implementation; speeds scans by adding indexes to regions rather secondary tables
Date Thu, 10 Dec 2009 19:28:18 GMT
Alternate indexed hbase implementation; speeds scans by adding indexes to regions rather secondary
tables
---------------------------------------------------------------------------------------------------------

                 Key: HBASE-2037
                 URL: https://issues.apache.org/jira/browse/HBASE-2037
             Project: Hadoop HBase
          Issue Type: New Feature
            Reporter: stack
             Fix For: 0.20.3


Purpose

The goal of the indexed HBase contrib is to speed up scans by indexing HBase columns. Indexed
HBase (IHbase) is different from the indexed tables in transactional HBase (ITHbase): while
the indexes in ITHBase are, in fact, hbase tables using the indexed column's values as row
keys, IHbase creates indexes at the region level. The differences are summarized in below.

+ global ordering
ITHBase: yes
IHBase: no
Comment: IHBase has an index for each region. The flip side of not having global ordering
is compatibility with the good old HRegion: results are coming back in row order (and not
value order as in THBase)

+ Full table scan?
ITHBase: no
IHBase: no
Comment: ITHbase does a partial scan on the index table. IHbase supports specifying start/end
rows to limit the number of scanned regions

+ Multiple Index Usage
ITHBase: no
IHBase: yes
Comment: IHBase can take advantage of multiple indexes in the same scan. IHBase IdxScan object
accepts an Expression which allows intersection/ unison of several indexed 
column criteria

+ Extra disk storage
ITHBase: yes
IHBase: no
Comment: IHbase indexes are created when the region starts/flushes and do not require any
extra storage

+ Extra RAM
ITHBase: yes
IHBase: yes
Comment: IHbase indexes are in memory and hence increase the memory overhead. THbase indexes
increase the number of regions each region server has to support thus costing memory too

+ Parallel scanning support
ITHBase: no
IHBase: yes
In ITHbase the index table needs to be consulted and then GETs are issued for each matching
row. The behavior of IHBase (as perceived by the client) is no different than a regular scan
and hence supports parallel scanning seamlessly. parallel GET can be implemented to speedup
ITHbase scans


Why IHbase should outperform ITHBase
1. More flexible: a. Supports range queries and multi-index queries b. Supports different
types - not only byte arrays
2. Less overhead: ITHbase pays at least two 'table roundtrips' - one for the index table and
the other for the main table
3. Quicker index expression evaluation: IHBase is using dedicated index data structures while
ITHbase is using the regular HRegion scan facilities

Implementation notes
• Only index Storefiles.Every index scan performs a full memstore scan. Indexing the memstore
will be implemented only if scanning the memstore will prove to be a performance bottleneck
• Index expression evaluation is performed using bit sets.There are two types of bitsets:
compressed and expanded. An index will typically store a compressed bitset while an expression
evaluator will most probably use an expanded bitset
+ TODO

This patch changes some some of hbase core so can instantiate other than default HRegion.
 Fixes bugs in filter too.

Would like to add this as a contrib. package on 0.20 branch in time for 0.20.3 if possible.

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