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# Efficient Queries Across Distributed Tables

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Photo by Eric McCowan

For distributed databases, efficient processing and transfer of records and values is paramount. Semijoins allow for data to be sent back and forth in a way that sounds like more work, but may in fact have less data transfer than an equivalent traditional inner join.

A database of athlete details has been vertically fragmented into two tables, ATHLETE_V1 and ATHLETE_V2 for efficient storage at two different sites, USER1_VP and USER2_VP. If we needed to join them together to access a large subset of the fields and run queries, we would typically run something like this (note we are using letter aliases b and c for our tables):

    select b.AthleteID, b.FName, b.SName, c.BDate, c.CCode, c.SportID
from "USER1_VP"."ATHLETE_V1" b, "USER2_VP"."ATHLETE_V2" c
where b.AthleteID=c.AthleteID;


From here, we can filter to get results like where CCODE='AUS' to find Australians. However, this can be broken up and done more efficiently, with far less data transfer and much faster speed, if we use semijoins and reorder our query. If our goal is to have a table containing all the Australian athletes, we probably don't need to access all of the records in both tables in the first place.

1. Get all the Athletes whose nationality is Australia from ATHLETE_V2:
    select AthleteID
from "USER2_VP"."ATHLETE_V2"
where CCODE='AUS';

1. Use this list of Australian athlete IDs to make a query to select all the related data from ATHLETE_V1 (note that we are nesting the first query):
    select *
from "USER1_VP"."ATHLETE_V1" a
where a.AthleteID in
(select AthleteID
from "USER2_VP"."ATHLETE_V2"
where CCODE='AUS');

1. This intermediate table contains IDs from ATHLETE_V2 and all fields from ATHLETE_V1. We can use it to get the rest of the fields from ATHLETE_V2:
    select b.AthleteID, b.FName, b.SName, c.BDate, c.CCode, c.SportID
from "USER2_VP"."ATHLETE_V2" c,
(select *
from "USER1_VP"."ATHLETE_V1" a
where a.AthleteID in
(select AthleteID
from "USER2_VP"."ATHLETE_V2"
where CCODE='AUS')) b
where b.AthleteID=c.AthleteID;


This final query with nesting provides the result we need, but due to only sending around and processing the records we needed, the network cost and time has been reduced significantly. As mentioned earlier this may not always be the case, and if possible it should be tested both ways first. It depends heavily on how the database fragmentation has has been designed. This approach can also be used when one site is slower but there is no replication of the fragments.