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 backwards 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 (initdb),
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 sqrt function:
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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