Semi-join Subquery Optimizations

MariaDB has a set of optimizations specifically targeted at semi-join subqueries.

What is a semi-join subquery

A semi-join subquery has a form of

SELECT ... FROM outer_tables WHERE expr IN (SELECT ... FROM inner_tables ...) AND ...

that is, the subquery is an IN-subquery and it is located in the WHERE clause. The most important part here is

with semi-join subquery, we're only interested in records of outer_tables that have matches in the subquery

Let's see why this is important. Consider a semi-join subquery:

select * from Country 
where 
  Continent='Europe' and 
  Country.Code in (select City.country 
                   from City 
                   where City.Population>1*1000*1000);

One can execute it "naturally", by starting from countries in Europe and checking if they have populous Cities:

semi-join-outer-to-inner

The semi-join property also allows "backwards" execution: we can start from big cities, and check which countries they are in:

semi-join-inner-to-outer

To contrast, let's change the subquery to be non-semi-join:

select * from Country 
where 
   Country.Continent='Europe' and 
   (Country.Code in (select City.country 
                   from City where City.Population>1*1000*1000) 
    or Country.SurfaceArea > 100*1000  -- Added this part
   );

It is still possible to start from countries, and then check

  • if a country has any big cities
  • if it has a large surface area:

non-semi-join-subquery

The opposite, city-to-country way is not possible. This is not a semi-join.

Difference from inner joins

Semi-join operations are similar to regular relational joins. There is a difference though: with semi-joins, you don't care how many matches an inner table has for an outer row. In the above countries-with-big-cities example, Germany will be returned once, even if it has three cities with populations of more than one million each.

Semi-join optimizations in MariaDB

MariaDB uses semi-join optimizations to run IN subqueries starting from MariaDB 5.3. Starting in MariaDB 5.3.3, Semi-join subquery optimizations are enabled by default. You can disable them by turning off their optimizer_switch like so:

SET optimizer_switch='semijoin=off'

MariaDB has five different semi-join execution strategies:

See Also

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