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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
uint16t_infomask2; // #attributes + flags
uint16t_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_HASNULL0x0001
/* has null attribute(s) */
#define HEAP_HASVARWIDTH0x0002
/* has variable-width attribute(s) */
#define HEAP_HASEXTERNAL0x0004
/* 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
booltdhasoid;// 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
{
Oidattrelid;// OID of reln containing attr
NameData attname; // name of attribute
Oidatttypid;// OID of attributes data type
int16attlen;// attribute length
int32attndims;// # 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 COMP9315 21T1 PostgreSQL Tuples [12/12]Produced: 28 Feb 2021