Python wrapper for MultiJoin feature

[lbooker42]

The call syntax is the following:

from deephaven.table import MultiJoinInput, MultiJoinTable, multi_join

# complex join
mj_input = [
    MultiJoinInput(table=t1, on="key"), # all columns added
    MultiJoinInput(table=t2, on="key=otherKey", joins=["col1", "col2"]), # specific columns added
]
multitable = multi_join(input=mj_input)

#simple joins
multitable = multi_join(tables=[t1,t2], on="common_key") # all columns from t1,t2 included in output
multitable = multi_join(tables=[t1,t2], on=["common_key1", "common_key2"]) # all columns from t1,t2 included

Closes #4280

[jmao-denver]

Good question!

1. Based on the precedents of PartitionedTable, TreeTable, RollupTable, it does seem to make sense to place MultiJoinTable in table.py
2. I am not sure the level of 'table-ness' MultiJoinTable is. My guess is that it is at the same level as the above mentioned ones. The only difference is that those are created by methods on Table, not unbound functions. In a way, it is more similar to table_factory.merge().
3. One of the reasons we put PartitionedTable etc. in table.py is to avoid cyclical imports. MultiJoinTable doesn't seem to have this problem.

Although a bit torn, my personal preference is for consistency by having MultiJoinTable in table.py. Maybe we can move the function multi_join' to table_factory`?

[jmao-denver]

Please move multi_join to table.py

[jmao-denver]

LGTM

[chipkent]

General API question. I don't know the answer, but I'm asking it....

Is there any reason the user would need to know about MultiJoinInputTable? Right now, the query syntax looks like:

t = multi_join([t1,t2,t3]).table()

Any reason it shouldn't just be

t = multi_join([t1,t2,t3])

[lbooker42]

Yes, but only because in the future we intend to allow addition (and potentially removal) of tables from this join.

mt = multi_join([t1,t2,t3]);
mt_new = mt.add_table(t4) # returns Immutable MultiJoinTable

[deephaven-internal]

Labels indicate documentation is required. Issues for documentation have been opened:

How-to: https://github.com/deephaven/deephaven.io/issues/3127
Reference: https://github.com/deephaven/deephaven.io/issues/3128