Mitesh Shah writes:
> One follow up question. In the example given, it says there are 36
> bytes for each row header and 4 bytes for each pointer to a tuple. I'm
> not sure where these numbers (36 and 4) are coming from. Are they
> standard for *every* table? If my table has more than just two
> integers, for example, will each row header be more than 36 bytes?
More or less. Quoth the source:
typedef struct HeapTupleHeaderData
{ Oid t_oid; /* OID of this tuple -- 4 bytes */
CommandId t_cmin; /* insert CID stamp -- 4 bytes each */ CommandId t_cmax;
/* delete CommandId stamp */
TransactionId t_xmin; /* insert XID stamp -- 4 bytes each */ TransactionId t_xmax; /*
deleteXID stamp */
ItemPointerData t_ctid; /* current TID of this or newer tuple */ int16 t_natts;
/* number of attributes */
uint16 t_infomask; /* various infos */
uint8 t_hoff; /* sizeof() tuple header */
/* ^ - 31 bytes - ^ */
bits8 t_bits[MinHeapTupleBitmapSize / 8]; /* bit map of NULLs */
/* MORE DATA FOLLOWS AT END OF STRUCT */
} HeapTupleHeaderData;
Most of the fields are for maintaining information required for
transaction rollback and multi-version concurrency control, in case you
can't quite decode it. ;-)
--
Peter Eisentraut peter_e@gmx.net http://yi.org/peter-e/
