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From stack <st...@duboce.net>
Subject Re: Secondary indexes
Date Tue, 22 Apr 2008 18:58:20 GMT
Some questions interlaced below:

Clint Morgan wrote:
> All,
> We want to put secondary indexes into hbase. The motivation is that we
> are storing data in hbase that we want to serve to users. We would
> like to be able to serve rows sorted by column values. Our queries
> will be over rows with a given key prefix, so we should not be hitting
> to many regions.
> I was thinking it would work roughly like this:
> - At table creation time, individual columns can be declared as
> indexed. By default we could sort the column values lexicographically,
> or we can provide a WritableComparatorFactory<T> which has the ability
> to make values of type T from a byte [], as well as providing a
> Comparator<T>. (Better than providing a Comparator<byte[]> as it only
> costs once per row insert for deserialization, rather that twice on
> each comparison).

I don't follow what the Factory adds.

We're talking about getting HBASE-82 into 0.2.  Does that interfere with 
this proposal?  (I'm thinking that along w/ rows becoming byte arrays 
rather Text with a client-supplied Comparator, column qualifiers would 
shift to be byte arrays also -- though yeah, implies that if your sort 
is not byte-lexicographical, yes, the compares can be costly involving 
two deserializations per compare).
> - We catch all writes/deletes and maintain a SortedMap<T, HStoreKey>
> which keeps the column values in order, and maps them back to row
> keys. First cut may just keep all this in memory, but it should be
> backed with MapFile(s).

Would be sweet if you could leverage the HBase memcache code and flusher 
to do the above.

This Map would be global for the table?  Or per Region?

A lucene index wouldn't work for you because you want ordering?

> - Add to the hregion the ability to scan through keys in column order.
> Just iterate through the SortedMap, run a filter on the key, and if it
> passes do a get on the row.

You'd be random reading rows.  You're OK w/ current performance?  (For 
sure it will only improve but....).

> - Add a ColumnOrderedClientScanner which will open column order
> scanners to all applicable hregions, and continuously pick row with
> the lowest column value from each of the client scanners.

This scanner would have a significant client-side component to do the 
arbitrage between all regions to figure the lowest column value?  If you 
had a new type of 'region' -- one denoted by lowest and upper column 
then the client-side logic would fade away and your scanner would look 
like current scanners.
> - Region splits should be easy enough, just a scan through the
> SortedMap to partition.

Splits would not be row-based and run as they currently do, but rather 
sorted-column based?

> Of course, the index could also be used for more efficient querying on
> the indexed column's values.
> Do other users have a need for this functionality?
> What do developers think about this? I know hbase is more intended for
> back-end batch style processing, but we have this need.
How are you thinking of adding in this new functionality?  Subclassing 


> Cheers,
> -clint

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