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PostgreSQL Tuples (cont)

Tuple-related data types: (cont) 

typedef struct HeapTupleFields  // simplified
{
  TransactionId t_xmin;  // inserting xact ID
  TransactionId t_xmax;  // deleting or locking xact ID
  union {
    CommandId   t_cid;   // inserting or deleting command ID
    TransactionId t_xvac;// old-style VACUUM FULL xact ID 
  } t_field3;
} HeapTupleFields;

Note that not all system fields from stored tuple appear

  • oid is stored after the tuple header, if used
  • both xmin/xmax are stored, but only one of cmin/cmax