Changed age_id, age_start_id and age_end_id function signatures to IM…

…MUTABLE (#405)

* Changed some function signatures to IMMUTABLE

Changed age_id(agtype), age_start_id(agtype) and age_end_id(agtype) from STABLE to IMMUTABLE.

* Revert "Changed some function signatures to IMMUTABLE"

This reverts commit 92ac40d.

* Changed age_id signature to IMMUTABLE

* Function signature was changed from STABLE to IMMUTABLE;
* Function receives a vertex or edge container (or null), and analyze its contents to return the "id" stored in said container;
* As it acts only on its arguments, it will always return the same result, given the same arguments, even across multiple SQL statements, being classified as IMMUTABLE function.

* Changed age_start_id signature to IMMUTABLE

* Function signature was changed from STABLE to IMMUTABLE;
* Function receives an edge container (or null), and analyze its contents to return the "start_id" stored in said container;
* As it acts only on its arguments, it will always return the same result, given the same arguments, even across multiple SQL statements, being classified as IMMUTABLE function.

* Changed age_end_id signature to IMMUTABLE

* Function signature was changed from STABLE to IMMUTABLE;
* Function receives an edge container (or null), and analyze its contents to return the "end_id" stored in said container;
* As it acts only on its arguments, it will always return the same result, given the same arguments, even across multiple SQL statements, being classified as IMMUTABLE function.

age--1.1.1.sql

@@ -3413,23 +3413,23 @@ AS 'MODULE_PATHNAME';
 CREATE FUNCTION ag_catalog.age_id(agtype)
 RETURNS agtype
 LANGUAGE c
-STABLE
+IMMUTABLE
 RETURNS NULL ON NULL INPUT
 PARALLEL SAFE
 AS 'MODULE_PATHNAME';
 
 CREATE FUNCTION ag_catalog.age_start_id(agtype)
 RETURNS agtype
 LANGUAGE c
-STABLE
+IMMUTABLE
 RETURNS NULL ON NULL INPUT
 PARALLEL SAFE
 AS 'MODULE_PATHNAME';
 
 CREATE FUNCTION ag_catalog.age_end_id(agtype)
 RETURNS agtype
 LANGUAGE c
-STABLE
+IMMUTABLE
 RETURNS NULL ON NULL INPUT
 PARALLEL SAFE
 AS 'MODULE_PATHNAME';