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HBase >> mail # dev >> Simple stastics per region

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Simple stastics per region
This topic comes up now and then (see recent discussion about translating multi Gets into Scan+Filter).

It's not that hard to keep statistics as part of compactions.
I envision two knobs:
1. Max number of distinct values to track directly. If a column has less this # of values, keep track of their occurrences explicitly.
2. Number of (equal width) histogram partitions to maintain.

Statistics would be kept per store (i.e. per region per column family) and stored into an HBase table (one row per store).Initially we could just support major compactions that atomically insert a new version of that statistics for the store.

An simple implementation (not knowing ahead of time how many values it will see during the compaction) could start by keeping track of individual values for columns. If it gets past the max # of distinct values to track, start with equal width histograms (using the distinct values picket up so far to estimate an initial partition width).
If the number of partition gets larger than what was configured it would increase the width and merge the previous counts into the new width (which means the new partition width must be a multiple of the previous size).
There's probably a lot of other fanciness that could be used here (haven't spent a lot of time thinking about details).
Is this something that should be in core HBase or rather be implemented as coprocessor?
-- Lars