首先看一下metadata cache中metadata-related objects对象的Base接口IMDInterface，metadata中所有对象的父类，代码位于src/backend/gporca/libnaucrates/include/naucrates/md/IMDInterface.h文件中。
class IMDInterface : public CRefCount{
public: virtual ~IMDInterface(){}
};再看一下metadata cache中metadata cache objects对象的Base接口IMDCacheObject，metadata cache中所有对象的父类，代码位于src/backend/gporca/libnaucrates/include/naucrates/md/IMDCacheObject.h文件中。其包含了将metadata id elements列表序列化的函数SerializeMDIdList。metadata cache objects所包含的对象的种类，本文关注EmdtRel，即表对象。
class IMDCacheObject : public IMDInterface{
public:
// Serialize a list of metadata id elements using pstrTokenList as the root XML element for the list, and each metadata id is serialized in the form of a pstrTokenListItem element. The serialized information looks like this:
// <strTokenList>
// <strTokenListItem .../>...
// </strTokenList>
static void SerializeMDIdList(CXMLSerializer *xml_serializer, const IMdIdArray *mdid_array, const CWStringConst *strTokenList, const CWStringConst *strTokenListItem);
enum Emdtype{ // type of md object
EmdtRel, EmdtInd, EmdtFunc, EmdtAgg, EmdtOp, EmdtType, EmdtTrigger, EmdtCheckConstraint, EmdtRelStats, EmdtColStats, EmdtCastFunc, EmdtScCmp };
virtual IMDId *MDId() const = 0; // md id of cache object
virtual CMDName Mdname() const = 0; // cache object name
virtual Emdtype MDType() const = 0; // object type
virtual void Serialize(gpdxl::CXMLSerializer *) const = 0; // serialize object in DXL format
virtual const CWStringDynamic *GetStrRepr() const = 0; // DXL string representation of cache object
// serialize the metadata id information as the attributes of an element with the given name
virtual void SerializeMDIdAsElem(gpdxl::CXMLSerializer *xml_serializer, const CWStringConst *element_name, const IMDId *mdid) const;
};
typedef CDynamicPtrArray<IMDCacheObject, CleanupRelease> IMDCacheObjectArray;IMDRelation接口
IMDRelation接口定义在src/backend/gporca/libnaurates/include/naucrates/md/IMDRelation.h文件中，是metadata cache中relations相关的接口。从如下Erelstoragetype和Ereldistrpolicy可以看出，该接口和GPDB的表存储类型【RELSTORAGE_HEAP - stored on disk using heap storage. RELSTORAGE_AOROWS - stored on disk using append only storage. RELSTORAGE_AOCOLS - stored on dist using append only column storage. RELSTORAGE_VIRTUAL - has virtual storage, meaning, relation has no data directly stored forit (right now this relates to views and comp types). RELSTORAGE_EXTERNAL- stored externally using external tables. RELSTORAGE_FOREIGN - stored in another server.】和分布策略、分区策略是相关的，因此如果需要支持其他数据库，需要创建新的接口继承IMDCacheObject接口，并导入相关特性。
class IMDRelation : public IMDCacheObject{
public:
enum Erelstoragetype { // Storage type of a relation
ErelstorageHeap,ErelstorageAppendOnlyCols,ErelstorageAppendOnlyRows,ErelstorageAppendOnlyParquet,
ErelstorageExternal,ErelstorageVirtual,ErelstorageSentinel
};
enum Ereldistrpolicy{ // Distribution policy of a relation
EreldistrMasterOnly,EreldistrHash,EreldistrRandom,EreldistrReplicated,EreldistrSentinel
};
enum Erelpartitiontype{ // Partition type of a partitioned relation
ErelpartitionRange = 'r', ErelpartitionList = 'l'
};
protected: static CWStringDynamic *ColumnsToStr(CMemoryPool *mp, ULongPtrArray *colid_array); // serialize an array of column ids into a comma-separated string
public:
virtual Emdtype MDType() const { return EmdtRel; } // object type
virtual BOOL IsTemporary() const = 0; // is this a temp relation
virtual Erelstoragetype RetrieveRelStorageType() const = 0; // storage type (heap, appendonly, ...)
virtual Ereldistrpolicy GetRelDistribution() const = 0; // distribution policy (none, hash, random)
virtual ULONG ColumnCount() const = 0; // number of columns
virtual DOUBLE ColWidth(ULONG pos) const = 0; // width of a column with regards to the position
virtual BOOL HasDroppedColumns() const = 0; // does relation have dropped columns
virtual ULONG NonDroppedColsCount() const = 0; // number of non-dropped columns
virtual ULONG NonDroppedColAt(ULONG pos) const = 0; // return the position of the given attribute position excluding dropped columns
virtual ULONG GetPosFromAttno(INT attno) const = 0; // return the position of a column in the metadata object given the attribute number in the system catalog
virtual ULongPtrArray *NonDroppedColsArray() const = 0; // return the original positions of all the non-dropped columns
virtual ULONG SystemColumnsCount() const = 0; // number of system columns
virtual const IMDColumn *GetMdCol(ULONG pos) const = 0; // retrieve the column at the given position
virtual ULONG KeySetCount() const = 0; // number of key sets
virtual const ULongPtrArray *KeySetAt(ULONG pos) const = 0; // key set at given position
virtual ULONG DistrColumnCount() const = 0; // number of distribution columns
virtual const IMDColumn *GetDistrColAt(ULONG pos) const = 0; // retrieve the column at the given position in the distribution key for the relation
virtual IMDId *GetDistrOpfamilyAt(ULONG pos) const = 0;
virtual BOOL ConvertHashToRandom() const = 0; // return true if a hash distributed table needs to be considered as random
virtual BOOL HasOids() const = 0; // does this table have oids
virtual BOOL IsPartitioned() const = 0; // is this a partitioned table
virtual ULONG PartColumnCount() const = 0; // number of partition columns
virtual ULONG PartitionCount() const = 0; // number of partitions
virtual const IMDColumn *PartColAt(ULONG pos) const = 0; // retrieve the partition column at the given position
virtual CharPtrArray *GetPartitionTypes() const = 0; // retrieve list of partition types
virtual CHAR PartTypeAtLevel(ULONG pos) const = 0; // retrieve the partition type of the given partition level
virtual ULONG IndexCount() const = 0; // number of indices
virtual ULONG TriggerCount() const = 0; // number of triggers
virtual IMDId *IndexMDidAt(ULONG pos) const = 0; // retrieve the id of the metadata cache index at the given position
virtual BOOL IsPartialIndex(IMDId *mdid) const { return false; }; // check if index is partial given its mdid
virtual IMDId *TriggerMDidAt(ULONG pos) const = 0; // retrieve the id of the metadata cache trigger at the given position
virtual ULONG CheckConstraintCount() const = 0; // number of check constraints
virtual IMDId *CheckConstraintMDidAt(ULONG pos) const = 0; // retrieve the id of the check constraint cache at the given position
virtual IMDPartConstraint *MDPartConstraint() const = 0; // part constraint
// external partitions (for partitioned tables) 对应增加GetForeignPartitions
virtual IMdIdArray *GetExternalPartitions() const { return NULL; }
// contains any external partitions (for partitioned tables only) 对应增加HasForeignPartitions
BOOL HasExternalPartitions() const {
return (NULL != GetExternalPartitions() && GetExternalPartitions()->Size() > 0);
}
static const CWStringConst *GetDistrPolicyStr( Ereldistrpolicy rel_distr_policy){ // relation distribution policy as a string value
switch (rel_distr_policy){
case EreldistrMasterOnly: return CDXLTokens::GetDXLTokenStr(EdxltokenRelDistrMasterOnly);
case EreldistrHash: return CDXLTokens::GetDXLTokenStr(EdxltokenRelDistrHash);
case EreldistrRandom: return CDXLTokens::GetDXLTokenStr(EdxltokenRelDistrRandom);
case EreldistrReplicated: return CDXLTokens::GetDXLTokenStr(EdxltokenRelDistrReplicated);
default: return NULL;
}
}
// name of storage type 为该函数增加Foreign分支
static const CWStringConst *GetStorageTypeStr( IMDRelation::Erelstoragetype rel_storage_type){
switch (rel_storage_type){
case ErelstorageHeap: return CDXLTokens::GetDXLTokenStr(EdxltokenRelStorageHeap);
case ErelstorageAppendOnlyCols: return CDXLTokens::GetDXLTokenStr( EdxltokenRelStorageAppendOnlyCols);
case ErelstorageAppendOnlyRows: return CDXLTokens::GetDXLTokenStr( EdxltokenRelStorageAppendOnlyRows);
case ErelstorageAppendOnlyParquet: return CDXLTokens::GetDXLTokenStr( EdxltokenRelStorageAppendOnlyParquet);
case ErelstorageExternal: return CDXLTokens::GetDXLTokenStr(EdxltokenRelStorageExternal);
case ErelstorageVirtual: return CDXLTokens::GetDXLTokenStr(EdxltokenRelStorageVirtual);
default: return NULL;
}
}
BOOL IsAORowOrColTable() const {
Erelstoragetype st = RetrieveRelStorageType();
return st == ErelstorageAppendOnlyCols || st == ErelstorageAppendOnlyRows;
}
};
// common structure over relation and external relation metadata for index info
typedef CDynamicPtrArray<CMDIndexInfo, CleanupRelease> CMDIndexInfoArray;
