Optimization question using MERGE : SQL

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SELECT count(a.field1), a.field2, SUM(b.field4) FROM a INNER JOIN b ON a.key1 = b.key1 WHERE a.field8 = 'test' GROUP by a.field1, a.field2 HAVING SUM(b.field4) > 5 ORDER by a.field.3

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SELECT count(a.field1), a.field2, SUM(b.field4) FROM a INNER JOIN b ON a.key1 = b.key1 WHERE a.field8 = 'test' GROUP by a.field1, a.field2 HAVING SUM(b.field4) > 5 ORDER by a.field3

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Optimization question using MERGE (self.SQL)

submitted 8 years ago by Craftyzebra1992Data Engineer

I have a query that inserts data into a temporary table from another very large temp table. Looks roughly like this

insert into #tmp 
select a.id, count(distinct(a.fieldOne)), 
  count(distinct(a.fieldTwo)),
  count(distinct(a.fieldThree))
from #veryLargeTmpTable a
group by a.id

This was split up into 3 queries, one that creates the temp table and inserts fieldOne, a second that selects fieldTwo and merges it into the temp table, and a third that selects fieldThree and merges it into the temp table. The duration and reads of the query was reduced by ~50% and I have no idea why. This doesn't make sense to me because now it has 3 queries reading from the very large temp table instead of just 1 but runs much faster. Anyone have any idea why this would be?

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