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
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?
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
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
+ Extra disk storage
Comment: IHbase indexes are created when the region starts/flushes and do not require any extra storage
+ Extra RAM
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
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
• 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
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.