===================================================================================================
This document describes the type and source of all metadata used and requested from optimizer. GPDB
collects all these data using GPDB function wrappers. 
Copyright (c) 2015-Present VMware, Inc. or its affiliates.
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Metadata Type (naucrates/md/IMDId.h) 	|	Includes		|	Source
------------------------------------------------------------------------------------------------------------------------------	
	EmdidGPDB			|	Index			|  relcache	
					|	Type			|  typcache & CatCache & Catalog(pg_type)	
					|	Relation		|  relcache	
					|	Scalar operator		|  Catalog(pg_operator) & CatCache	
					|	Aggregate		|  Catalog(pg_proc, pg_aggregate) & CatCache	
					|	Function		|  Catalog(pg_proc) & CatCache	
					|	Trigger			|  Catalog(pg_trigger) & CatCache	
					|	Constratint		|  Catalog(pg_constraint) & CatCache	
-------------------------------------------------------------------------------------------------------------------------------
	EmdidRelStats			|	Relation Stats		|  relcache
-------------------------------------------------------------------------------------------------------------------------------		
	EmdidColStats			|	Column statistics	|  relcache & Catalog(pg_statistic, pg_type) & CatCache	
-------------------------------------------------------------------------------------------------------------------------------	
	EmdidCastFunc			|	Cast Function		|  Catalog(pg_cast, pg_proc) & CatCache	
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	EmdidScCmp			|  Scalar Comparison Function	|  Catalog(pg_amop, pg_operator) & CatCache	
