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Tuple Representation

•Tuples
•Records vs Tuples
•Converting Records to Tuples
•Operations on Records
•Operations on Tuples
•Fixed-length Records
•Variable-length Records
•Data Types
•Field Descriptors
COMP9315 21T1 ♢ Tuple Representation ♢ [0/13]
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❖ Tuples

Each page contains a collection of tuples


What do tuples contain? How are they structured internally?
COMP9315 21T1 ♢ Tuple Representation ♢ [1/13]
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❖ Records vs Tuples

A table is defined by a schema, e.g.

create table Employee (
id integer primary key,
name varchar(20) not null,
jobvarchar(10),
dept smallint references Dept(id)
);
where a schema is a collection of attributes  (name,type,constraints)
Reminder: schema information (meta-data) is also stored, in the DB catalog
COMP9315 21T1 ♢ Tuple Representation ♢ [2/13]
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❖ Records vs Tuples (cont)

Tuple = collection of attribute values based on a schema, e.g.


Record = sequence of bytes, containing data for one tuple, e.g.


Bytes need to be interpreted relative to schema to get tuple
COMP9315 21T1 ♢ Tuple Representation ♢ [3/13]
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❖ Converting Records to Tuples

A Record is an array of bytes (byte[])
•representing the data values from a typed Tuple
•stored on disk (persistent) or in a memory buffer
A Tuple is a collection of named,typed values  (cf. C struct)
•to manipulate the values, need an “interpretable” structure
•stored in working memory, and temporary



COMP9315 21T1 ♢ Tuple Representation ♢ [4/13]
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❖ Converting Records to Tuples (cont)

Information on how to interpret bytes in a record …
•may be contained in schema data in DBMS catalog
•may be stored in the page directory
•may be stored in the record (in a record header)
•may be stored partly in the record and partly in the schema
For variable-length records, some formatting info …
•must be stored in the record or in the page directory
•at the least, need to know how many bytes in each varlen value
COMP9315 21T1 ♢ Tuple Representation ♢ [5/13]
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❖ Operations on Records

Common operation on records … access record via RecordId:

Record get_record(Relation rel, RecordId rid) {
(pid,tid) = rid;
Page buf = get_page(rel, pid);
return get_bytes(rel, buf, tid);
}

Cannot use a Record directly; need a Tuple:

Relation rel = … // relation schema
Record rec = get_record(rel, rid)
Tuple t = mkTuple(rel, rec)
Once we have a Tuple, we can access individual attributes/fields
COMP9315 21T1 ♢ Tuple Representation ♢ [6/13]
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❖ Operations on Tuples

Once we have a record, we need to interpret it as a tuple …
Tuple t = mkTuple(rel, rec)
•convert record to tuple data structure for relation rel
Once we have a tuple, we want to examines its contents …
Typ   getTypField(Tuple t, int i)
•extract the i’th field from a Tuple as a value of type Typ
E.g.   int x = getIntField(t,1),   char *s = getStrField(t,2)
COMP9315 21T1 ♢ Tuple Representation ♢ [7/13]
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❖ Fixed-length Records

A possible encoding scheme for fixed-length records:
•record format (length + offsets) stored in catalog
•data values stored in fixed-size slots in data pages


Since record format is frequently used at query time, cache in memory.
COMP9315 21T1 ♢ Tuple Representation ♢ [8/13]
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❖ Variable-length Records

Possible encoding schemes for variable-length records:
•Prefix each field by length 

•Terminate fields by delimiter 

•Array of offsets 

COMP9315 21T1 ♢ Tuple Representation ♢ [9/13]
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❖ Data Types

DBMSs typically define a fixed set of base types, e.g.

DATE,  FLOAT,  INTEGER,  NUMBER(n),  VARCHAR(n), …
This determines implementation-level data types for field values:
DATE

time_t
FLOAT

float,double
INTEGER

int,long
NUMBER(n)

int[] (?)
VARCHAR(n)

char[]
PostgreSQL allows new base types to be added
COMP9315 21T1 ♢ Tuple Representation ♢ [10/13]
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❖ Field Descriptors

A Tuple could be implemented as
•a list of field descriptors for a record instance 
(where a FieldDesc gives (offset,length,type) information)
•along with a reference to the Record data

typedef struct {
ushortnfields; // number of fields/attrs
ushortdata_off;// offset in struct for data
FieldDesc fields[];// field descriptions
Recorddata;// pointer to record in buffer
} Tuple;
Fields are derived from relation descriptor + record instance data.
COMP9315 21T1 ♢ Tuple Representation ♢ [11/13]
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❖ Field Descriptors (cont)

Tuple data could be
•a pointer to bytes stored elsewhere in memory



COMP9315 21T1 ♢ Tuple Representation ♢ [12/13]
<< ∧ ❖ Field Descriptors (cont)Or, tuple data could be … •appended to Tuple struct   (used widely in PostgreSQL)  COMP9315 21T1 ♢ Tuple Representation ♢ [13/13]Produced: 27 Feb 2021