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Hive, mail # user - Override COUNT() function


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Override COUNT() function
Peter Marron 2013-06-28, 14:54
Hi,

I feel sure that someone has asked for this before, but here goes...

In the case where I have the query

                SELECT COUNT(*) FROM table;

There are many cases where I can determine the count immediately.
(For example if I have run something like:

ANALYZE TABLE tablename [PARTITION(partcol1[=val1], partcol2[=val2], ...)] COMPUTE STATISTICS [noscan];

then there seems to be a table property "numRows" which holds a count of the number of rows.
Now I know that the COUNT function can't always be determined easily.
If the query is more complicated, like

                SELECT COUNT(*) FROM table GROUP BY column;

then obviously a simple scalar count is of no real use. But is there some way
to intercept the simple case and avoid running a table scan?

One problem that I see is that the COUNT function is a UDAF and I am
assuming that the presence of any aggregate function like this is enough
to force the query planner to require a Map/Reduce. Is there anyway
to make the function look like a simple UDF for some queries? Or
just for some tables? I guess that I'd be prepared to sacrifice the full
generality of the normal COUNT function for one which
only functions correctly for the simple query on my tables.

So is it possible to have a different COUNT function only on certain tables?

Regards,

Z