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From rajeshbabu chintaguntla <rajeshbabu.chintagun...@huawei.com>
Subject Regarding support to make use of partial covered indexes in scan
Date Thu, 30 Jan 2014 15:27:44 GMT
Hi,

Normally when we want to use secondary indices we create index on one or very few
columns of interest in query conditions. Index may not contain all the columns to retrieve.

Currently Phoenix supporting full covered indexes only(where all or most of the columns
should be in the index in many cases). When we run a query we will choose to scan from user
table
or index table based on condition that  whether all projected columns in the index or not.
This approach may have some disadvantages mainly in case of wider tables.
1)If we did just store all the columns in the index, then it would be just like creating another
copy of the entire table
 – which would take up way too much space and would be very inefficient for wider tables.
2) Some times if user creates index on few columns and observes that index is not getting
used
and then he need to add all the projected columns to index(may be to the part of index or
included columns).
which is something like we are exposing design decisions to the users, especially when we
already
target to simplify user's experience by giving SQL on top of a noSQL DB.
3) One more thing is as of now if we have an index table contains all projected columns in
the
query then we are simply scanning index table only.This can also become full table scan when
we don't
have any condition in the query or condition on non primary key column of index.
Some times this might give bad performance than normal table full scan.

Any one working to make use of partial covered indexes to avoid full table scan?

We are interested to work on this in Phoenix.

As part of our local indexing solution, we have similar kind of support.
>From index table first we fetch main table row key and from that we get
projected columns data from main table so that we can skip most of records
to scan from main table.

Thanks,
Rajeshbabu.







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