Fix for Index Advisor related hooks

Looks like the function get_actual_variable_range() was written with the
knowledge that virtual/hypothetical indexes may exist, but the assumption
seems wrong.

One one hand get_actual_variable_range() expects that virtual indexes do not
have an OID assigned, on the other hand explain_get_index_name_hook() is
handed just an index's OID to get its name back; IMHO these are based on two
conflicting assumptions about whether a virtual index will have an OID
assigned.

Attached patch fix_get_actual_variable_range.patch tries to fix this by
introducing a new hook that can help Postgres decide if an index is
fictitious or not.

Also attached is the patch expose_IndexSupportInitialize.patch, that makes
the static function IndexSupportInitialize() global so that the Index
Advisor doesn't have to reinvent the wheel to prepare an index structure
with opfamilies and opclasses.

Regards,
--
gurjeet.singh
@ EnterpriseDB - The Enterprise Postgres Company
http://www.EnterpriseDB.com

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