keep rb_exsit definition for backward compatible with verion less tha…

[harry-leq]

Function rb_exsit is renamed to rb_exists in v1.0.

Even though there is an attempt to replace the function definition in roaringbitmap--0.5--1.0.sql for migration, it still cause problem if user specifically install with a version less than 1.0 as shown below:

postgres=# create extension roaringbitmap with version '0.2';
ERROR:  could not find function "rb_exsit" in file "/opt/homebrew/lib/postgresql@18/roaringbitmap.dylib"
CONTEXT:  SQL statement "CREATE OR REPLACE FUNCTION rb_contains(roaringbitmap, integer)
  RETURNS bool 
  AS '$libdir/roaringbitmap', 'rb_exsit'
  LANGUAGE C STRICT IMMUTABLE PARALLEL SAFE"
extension script file "roaringbitmap--0.2.sql", near line 93

This commit retains the definition of rb_exsit for backward compatibility that allow user to successfully create roaringbitmap with old version and migrate to newer version without error during installation or runtime.

postgres=# create extension roaringbitmap with version '0.2';
CREATE EXTENSION
postgres=# select * from pg_available_extensions where name = 'roaringbitmap';
     name      | default_version | installed_version |           comment           
---------------+-----------------+-------------------+-----------------------------
 roaringbitmap | 1.2             | 0.2               | support for Roaring Bitmaps
(1 row)

[ChenHuajun]

Why use a higher-version .so file to create a lower-version extension? This behavior is not supported.

[harry-leq]

While the default version of roaringbitmap listed in pg_available_extensions is 1.2, older versions are listed under pg_available_extension_versions as well.

postgres=# select * from pg_available_extensions where name = 'roaringbitmap';
     name      | default_version | installed_version |           comment
---------------+-----------------+-------------------+-----------------------------
 roaringbitmap | 1.2             |                   | support for Roaring Bitmaps
(1 row)

postgres=# select * from pg_available_extension_versions where name = 'roaringbitmap';
     name      | version | installed | superuser | trusted | relocatable | schema | requires |           comment
---------------+---------+-----------+-----------+---------+-------------+--------+----------+-----------------------------
 roaringbitmap | 0.2     | f         | t         | f       | t           |        |          | support for Roaring Bitmaps
 roaringbitmap | 0.3     | f         | t         | f       | t           |        |          | support for Roaring Bitmaps
 roaringbitmap | 0.4     | f         | t         | f       | t           |        |          | support for Roaring Bitmaps
 roaringbitmap | 0.5     | f         | t         | f       | t           |        |          | support for Roaring Bitmaps
 roaringbitmap | 1.0     | f         | t         | f       | t           |        |          | support for Roaring Bitmaps
 roaringbitmap | 1.1     | f         | t         | f       | t           |        |          | support for Roaring Bitmaps
 roaringbitmap | 1.2     | f         | t         | f       | t           |        |          | support for Roaring Bitmaps
(7 rows)

It might not be a good practice to install older version against later .so file. But the fact that old versions are listed in system catalogue implies that they are supported. This also happens in many other extensions. There are a couple of ways to fix it:

1. Prevent the older versions from being listed by removing the migration .sql files -> this might cause unforeseen problems when users perform upgrading.
2. Keep the migration files and maintain the backward compatibility of the function, which is done by this commit. Besides that, it's always a good practice for higher version to keep compatibility with lower as much as it can.