Hello everybody
We (at University of Zagreb and University of Washington) have
implemented an optimization of Spark's sort-merge join (SMJ) which has
improved performance of our jobs considerably and we would like to know
if Spark community thinks it would be useful to include this in the main
distribution.
The problem we are solving is the case where you have two big tables
partitioned by X column, but also sorted by Y column (within partitions)
and you need to calculate an expensive function on the joined rows.
During a sort-merge join, Spark will do cross-joins of all rows that
have the same X values and calculate the function's value on all of
them. If the two tables have a large number of rows per X, this can
result in a huge number of calculations.
Our optimization allows you to reduce the number of matching rows per X
using a range condition on Y columns of the two tables. Something like:
... WHERE t1.X = t2.X AND t1.Y BETWEEN t2.Y - d AND t2.Y + d
The way SMJ is currently implemented, these extra conditions have no
influence on the number of rows (per X) being checked because these
extra conditions are put in the same block with the function being
calculated.
Our optimization changes the sort-merge join so that, when these extra
conditions are specified, a queue is used instead of the
ExternalAppendOnlyUnsafeRowArray class. This queue is then used as a
moving window across the values from the right relation as the left row
changes. You could call this a combination of an equi-join and a theta
join (we call it "sort-merge inner range join").
Potential use-cases for this are joins based on spatial or temporal
distance calculations.
The optimization is triggered automatically when an equi-join expression
is present AND lower and upper range conditions on a secondary column
are specified. If the tables aren't sorted by both columns, appropriate
sorts will be added.
We have several questions:
1. Do you see any other way to optimize queries like these (eliminate
unnecessary calculations) without changing the sort-merge join algorithm?
2. We believe there is a more general pattern here and that this could
help in other similar situations where secondary sorting is available.
Would you agree?
3. Would you like us to open a JIRA ticket and create a pull request?
Thanks,
Petar Zecevic
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