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PostgreSQL Tuples
• PostgreSQL Tuples
• PostgreSQL Attribute Values
COMP9315 21T1 ♢ PostgreSQL Tuples ♢ [0/12]
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❖ PostgreSQL Tuples
Definitions: include/postgres.h, include/access/*tup*.h
Functions: backend/access/common/*tup*.c e.g.
• HeapTuple heap_form_tuple(desc,values[],isnull[])
• heap_deform_tuple(tuple,desc,values[],isnull[])
PostgreSQL implements tuples via:
• a contiguous chunk of memory
• starting with a header giving e.g. #fields, nulls
• followed by data values (as a sequence of Datum)
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❖ PostgreSQL Tuples (cont)
HeapTupleData contains information about a stored tuple
typedef HeapTupleData *HeapTuple;
typedef struct HeapTupleData
{
uint32 t_len; // length of *t_data
ItemPointerData t_self; // SelfItemPointer
Oid t_tableOid; // table the tuple came from
HeapTupleHeader t_data; // -> tuple header and data
} HeapTupleData;
HeapTupleHeader is a pointer to a location in a buffer
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❖ PostgreSQL Tuples (cont)
Structure of HeapTuple:

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❖ PostgreSQL Tuples (cont)
PostgreSQL stores each record as tuple header, followed by data:
typedef HeapTupleHeaderData *HeapTupleHeader;
typedef struct HeapTupleHeaderData // simplified
{
HeapTupleFields t_heap;
ItemPointerData t_ctid; // TID of newer version
uint16 t_infomask2; // #attributes + flags
uint16 t_infomask; // flags e.g. has_null
uint8 t_hoff; // sizeof header incl. t_bits
// above is fixed size (23 bytes) for all heap tuples
bits8 t_bits[1]; // bitmap of NULLs, var.len.
// OID goes here if HEAP_HASOID is set in t_infomask
// actual data follows at end of struct
} HeapTupleHeaderData;
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❖ PostgreSQL Tuples (cont)
Tuple structure:

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❖ PostgreSQL Tuples (cont)
Some of the bits in t_infomask ..
#define HEAP_HASNULL 0x0001
/* has null attribute(s) */
#define HEAP_HASVARWIDTH 0x0002
/* has variable-width attribute(s) */
#define HEAP_HASEXTERNAL 0x0004
/* has external stored attribute(s) */
#define HEAP_HASOID_OLD 0x0008
/* has an object-id field */
Location of NULLs is stored in t_bits[] array
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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
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❖ PostgreSQL Tuples (cont)
Tuple-related data types: (cont)
// TupleDesc: schema-related information for HeapTuples
typedef struct tupleDesc
{
int natts; // # attributes in tuple
Oid tdtypeid; // composite type ID for tuple type
int32 tdtypmod; // typmod for tuple type
bool tdhasoid; // does tuple have oid attribute?
int tdrefcount; // reference count (-1 if not counting)
TupleConstr *constr; // constraints, or NULL if none
FormData_pg_attribute attrs[];
// attrs[N] is a pointer to description of attribute N+1
} *TupleDesc;
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❖ PostgreSQL Tuples (cont)
Tuple-related data types: (cont)
// FormData_pg_attribute:
// schema-related information for one attribute
typedef struct FormData_pg_attribute
{
Oid attrelid; // OID of reln containing attr
NameData attname; // name of attribute
Oid atttypid; // OID of attribute’s data type
int16 attlen; // attribute length
int32 attndims; // # dimensions if array type
bool attnotnull; // can attribute have NULL value
….. // and many other fields
} FormData_pg_attribute;
For details, see include/catalog/pg_attribute.h
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❖ PostgreSQL Attribute Values
Attribute values in PostgreSQL tuples are packaged as Datums
// representation of a data value
typedef uintptr_t Datum;
The actual data value:
• may be stored in the Datum (e.g. int)
• may have a header with length (for varlen attributes)
• may be stored in a TOAST file (if large value)
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❖ PostgreSQL Attribute Values (cont)
Attribute values can be extracted as Datum from HeapTuples
Datum heap_getattr(
HeapTuple tup, // tuple (in memory)
int attnum, // which attribute
TupleDesc tupDesc, // field descriptors
bool *isnull // flag to record NULL
)
isnull is set to true if value of field is NULL
attnum can be negative … to access system attributes (e.g. OID)
For details, see include/access/htup_details.h
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❖ PostgreSQL Attribute Values (cont)
Values of Datum objects can be manipulated via e.g.
// DatumGetBool:
// Returns boolean value of a Datum.
#define DatumGetBool(X) ((bool) ((X) != 0))
// BoolGetDatum:
// Returns Datum representation for a boolean.
#define BoolGetDatum(X) ((Datum) ((X) ? 1 : 0))
For details, see include/postgres.h
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Produced: 28 Feb 2021