Internal functions are functions written in C that have been statically linked into the PostgreSQL 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 (see
Section 17.2), but a user
could use CREATE FUNCTION to create
additional alias names for an internal function. Internal
functions are declared in CREATE
FUNCTION with language name internal. For instance, to create an alias for the
CREATE FUNCTION square_root(double precision) RETURNS double precision AS 'dsqrt' LANGUAGE internal STRICT;
(Most internal functions expect to be declared "strict".)
Note: Not all "predefined" functions are "internal" in the above sense. Some predefined functions are written in SQL.
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