On Jun 16, 2010, at 8:36 AM, Gang Luo wrote:
Hi,
there is something confusing me in the skew join (http://wiki.apache.org/pig/PigSkewedJoinSpec
)
1. does the sampling job sample and build histogram on both tables,
or just one table (in this case, which one) ?
Just the left one.
2. the join job still take the two table as inputs, and shuffle
tuples from partitioned table to particular reducer (one tuple to
one reducer), and shuffle tuples from streamed table to all reducers
associative to one partition (one tuple to multiple reducers). Is
that correct?
Keys with small enough values to fit in memory are shuffled to
reducers as normal. Keys that are too large are split between
reducers on the left side, and replicated to all of those reducers
that have the splits (not all reducers) on the right side. Does that
answer your question?
3. Hot keys need more than one reducers. Are these reducers
dedicated to this key only? Could they also take other keys at the
same time?
They take other keys at the same time.
4. for non-hot keys, my understanding is that they are shuffled to
reducers based on default hash partitioner. However, it could happen
all the keys shuffled to one reducers incurs skew even none of them
is skewed individually.
This is always the case in map reduce, though a good hash function
should minimize the occurrences of this.
Can someone give me some ideas on these? Thanks.
-Gang
Alan.
