PostgreSQL数据库RelationAM——RelationData

RelationData是访问引擎中最重要的一个结构体，贯穿整个Relation访问的流程。const struct TableAmRoutine *rd_tableam用于存放该表的访问函数的结构体指针，见PostgreSQL数据库TableAM——Table Access Method。struct IndexAmRoutine *rd_indam;用于存放该表的索引访问函数的结构体指针。

typedef struct RelationData {
	RelFileNode rd_node;		
	
	struct SMgrRelationData *rd_smgr;	
	int			rd_refcnt;		
	BackendId	rd_backend;		
	bool		rd_islocaltemp; 
	bool		rd_isnailed;	
	bool		rd_isvalid;		
	bool		rd_indexvalid;	
	bool		rd_statvalid;	
	
	SubTransactionId rd_createSubid;	
	SubTransactionId rd_newRelfilenodeSubid;	

	Form_pg_class rd_rel;		
	TupleDesc	rd_att;			
	Oid			rd_id;			
	LockInfoData rd_lockInfo;	
	RuleLock   *rd_rules;		
	MemoryContext rd_rulescxt;	
	TriggerDesc *trigdesc;		
	
	struct RowSecurityDesc *rd_rsdesc;	

	
	List	   *rd_fkeylist;	
	bool		rd_fkeyvalid;	

	struct PartitionKeyData *rd_partkey;	
	MemoryContext rd_partkeycxt;	
	struct PartitionDescData *rd_partdesc;	
	MemoryContext rd_pdcxt;		
	List	   *rd_partcheck;	
	bool		rd_partcheckvalid;	
	MemoryContext rd_partcheckcxt;	

	
	List	   *rd_indexlist;	
	Oid			rd_pkindex;		
	Oid			rd_replidindex; 

	
	List	   *rd_statlist;	

	
	Bitmapset  *rd_indexattr;	
	Bitmapset  *rd_keyattr;		
	Bitmapset  *rd_pkattr;		
	Bitmapset  *rd_idattr;		

	PublicationActions *rd_pubactions;	

	
	bytea	   *rd_options;		

	
	Oid			rd_amhandler;	

	
	const struct TableAmRoutine *rd_tableam;

	
	Form_pg_index rd_index;		
	
	struct HeapTupleData *rd_indextuple;	

	
	MemoryContext rd_indexcxt;	
	
	struct IndexAmRoutine *rd_indam;	
	Oid		   *rd_opfamily;	
	Oid		   *rd_opcintype;	
	RegProcedure *rd_support;	
	FmgrInfo   *rd_supportinfo; 
	int16	   *rd_indoption;	
	List	   *rd_indexprs;	
	List	   *rd_indpred;		
	Oid		   *rd_exclops;		
	Oid		   *rd_exclprocs;	
	uint16	   *rd_exclstrats;	
	Oid		   *rd_indcollation;	

	
	void	   *rd_amcache;		

	

	
	struct FdwRoutine *rd_fdwroutine;	

	
	Oid			rd_toastoid;	

	
	struct PgStat_TableStatus *pgstat_info; 
} RelationData;

src/backend/utils/cache/relcache.c
RelationIdGetRelation首先从cache中查看是否已经有相应的reldexc存在，则返回。如果不存在，调用RelationBuildDesc在cache中创建一个并返回。

Relation RelationIdGetRelation(Oid relationId) {
	Relation	rd;
	
	Assert(IsTransactionState());
	
	RelationIdCacheLookup(relationId, rd);
	if (RelationIsValid(rd)){
		RelationIncrementReferenceCount(rd);
		
		if (!rd->rd_isvalid){
			
			if (rd->rd_rel->relkind == RELKIND_INDEX || rd->rd_rel->relkind == RELKIND_PARTITIONED_INDEX)
				RelationReloadIndexInfo(rd);
			else
				RelationClearRelation(rd, true);
			
			Assert(rd->rd_isvalid || (rd->rd_isnailed && !criticalRelcachesBuilt));
		}
		return rd;
	}

	
	rd = RelationBuildDesc(relationId, true);
	if (RelationIsValid(rd)) RelationIncrementReferenceCount(rd);
	return rd;
}