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From "He Yongqiang (JIRA)" <>
Subject [jira] Commented: (HIVE-588) LIMIT n is slower than it needs to be
Date Wed, 15 Jul 2009 05:25:14 GMT


He Yongqiang commented on HIVE-588:

>>when any mapper has printed 10 rows, Hive should print those 10 rows and kill the
rest of the job
it is not easy to implement with current hadoop map-reduce. You can not kill a whole job inside
some mapper. 
And hive does not know when enough rows have been collected. 
Even though hive use a done flag to note the mapper's job is finished, the split scan for
that mapper is not stopped. And new kv pairs are fed in to the mapper and are simply discarded
when the done flag in mapper is set. 

I think a workaroud would be:
1) file an issue in mapreduce to add the ability to let a mapper and thus maprunner terminate
itself when some done flag is set
2) extend a maprunner in hive to do the same, because issue in map-reduce may need a long
time to review, commit, release etc.

Even though the ability of terminating mapper is added, we can not share a global flag among
all mappers. In order to do that, i think one potential method would be to narrow the mappers'
number and thus a larger split is fed for each mapper (we will only need to touch a small
piece of the whole split).  

> LIMIT n is slower than it needs to be
> -------------------------------------
>                 Key: HIVE-588
>                 URL:
>             Project: Hadoop Hive
>          Issue Type: Improvement
>            Reporter: Adam Kramer
> ...simply prints the output of the first 10 lines of the first file in the database.
That's good.
> However,
> SELECT function(a) FROM t LIMIT 10;
> appears to send all of t to the mappers, runs the function, and and then returns the
first 10 rows from whatever mapper(s) finish first. This is very slow in some cases!
> Appropriate behavior for LIMIT would be to use ONE mapper, and to push files from the
table into that mapper, and then auto-kill the mapper once it has output 10 rows...just take
the first 10 rows and kill the whole task if necessary. On dying, throw some informative error
message like, "Dying intentionally; LIMIT has been reached." This should be the case even
for TRANSFORMs in the mapper...the TRANSFORM could spit out 20 rows, but once it has split
out 10, the whole task should die and the 10 should be returned immediately.
> The purpose of LIMIT is not just to have "only one response," but it's also to speed
up queries a whole lot. Running the function over the entire table is a big waste.
> Obviously, when a reduce step is necessary, the whole table will have to be pushed through
mappers and then copied and then sorted--but in those cases, whenever 10 total rows have been
output by any reducer(s), at which point all reduce tasks should be killed.

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