Thanks Navis, This is a very interesting class which I feel pretty sure that I would never have found. Are there any descriptions, motivations, documentation or examples anywhere? I suspect that there's nothing other than the source itself, but I had to ask.
Regards, Z -----Original Message----- From: Navis류승우 [mailto:navis....@nexr.com] Sent: 02 July 2013 08:50 To: user@hive.apache.org Subject: Re: Override COUNT() function MetadataOnlyOptimizer changes GBY on partition columns to simple TableScan with one line dummy. I think similar things can be done with stats. 2013/6/28 Peter Marron <peter.mar...@trilliumsoftware.com>: > 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 > >
