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Internal functions are functions written in C that have been statically linked into thePostgreSQL server. The“body” of the function definition specifies the C-language name of the function, which need not be the same as the name being declared for SQL use. (For reasons of backward compatibility, an empty body is accepted as meaning that the C-language function name is the same as the SQL name.)

Normally, all internal functions present in the server are declared during the initialization of the database cluster (seeSection 18.2), but a user could useCREATE FUNCTION to create additional alias names for an internal function. Internal functions are declared inCREATE FUNCTION with language nameinternal. For instance, to create an alias for thesqrt function:

CREATE FUNCTION square_root(double precision) RETURNS double precision    AS 'dsqrt'    LANGUAGE internal    STRICT;

(Most internal functions expect to be declared“strict”.)