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Tux Machines (TM)-specific
Since version 8.4, PostgreSQL has been offering a new optimisation strategy for the optimisation of certain queries: Semi and Anti Joins.
A Semi Join is a specific form of a join, which only takes the keys of relation a into account if these are also present in the associated table b. An Anti Join is the negative form of a Semi Join: that is, a key picked in table a will be taken into account if it is not present in table b.
To summarize, Semi and Anti Joins are specific forms of a join which only take certain keys on the left side into account - where queries want to make sure certain keys exist, but are not concerned with the content of the key itself. This behaviour is already widely known in Object Relation Mappers (ORM) which formulate such queries using EXIST() or NOT EXIST().