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FirstMatch strategy

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The idea

FirstMatch is an execution strategy for Semi-join subqueries.

It is very similar to how IN/EXISTS subqueries were executed in MySQL 5.x.

Let's take the usual example of search for countries with big cities:

select * from Country 
where Country.code IN (select City.Country 
                       from City 
                       where City.Population > 1*1000*1000)
      and Country.continent='Europe'

Suppose, our execution plan is to find countries in Europe, and then for each found country check if it has any big cities. Regular inner join execution will look as follows:

firstmatch-inner-join

Since Germany has got two big cities (in this diagram), it will be put into query output two times. FirstMatch strategy solves this by short-cutting execution as soon as the first match was found:

firstmatch-firstmatch

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