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Catalogs

Database Objects
Catalogs
Representing Databases
Representing Tables
COMP9315 21T1 Catalogs [0/11]
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Database Objects

RDBMSs manage different kinds of objects
databases, schemas, tablespaces
relations/tables, attributes, tuples/records
constraints, assertions
views, stored procedures, triggers, rules
Many objects have names (and, in PostgreSQL, some have OIDs).
How are the different types of objects represented?
How do we go from a name (or OID) to bytes stored on disk?
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Catalogs

Consider what information the RDBMS needs about relations:
name, owner, primary key of each relation
name, data type, constraints for each attribute
authorisation for operations on each relation
Similarly for other DBMS objects (e.g. views, functions, triggers, )
This information is stored in the system catalog tables
Standard for catalogs in SQL:2003: INFORMATION_SCHEMA
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Catalogs (cont)

The catalog is affected by several types of SQL operations:
create Object as Definition
drop Object
alter Object Changes
grant Privilege on Object
where Object is one of table, view, function, trigger, schema,
E.g. drop table Groups; produces something like

delete from Tables
whereschema = public and name = groups;

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Catalogs (cont)

In PostgreSQL, the system catalog is available to users via:
special commands in the psql shell (e.g. d)
SQL standard information_schema e.g. select * from information_schema.tables;

The low-level representation is available to sysadmins via:
a global schema called pg_catalog
a set of tables/views in that schema (e.g. pg_tables)
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Catalogs (cont)

You can explore the PostgreSQl catalog via psql commands
d gives a list of all tables and views
d Table gives a schema for Table
df gives a list of user-defined functions
df+ Function gives details of Function
ef Function allows you to edit Function
dv gives a list of user-defined views
d+ View gives definition of View
You can also explore via SQL on the catalog tables
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Catalogs (cont)

A PostgreSQL installation (cluster) typically has many DBs
Some catalog information is global, e.g.
catalog tables defining: databases, users,
one copy of each such table for the whole PostgreSQL installation
shared by all databases in the cluster (in PGDATA/pg_global)
Other catalog information is local to each database, e.g
schemas, tables, attributes, functions, types,
separate copy of each local table in each database
a copy of many global tables is made on database creation
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Catalogs (cont)

Side-note: PostgreSQL tuples contain
owner-specified attributes (from create table)
system-defined attributes
oid

unique identifying number for tuple (optional)
tableoid

which table this tuple belongs to
xmin/xmax

which transaction created/deleted tuple (for MVCC)

OIDs are used as primary keys in many of the catalog tables.
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Representing Databases

Above the level of individual DB schemata, we have:
databases represented by pg_database
schemas represented by pg_namespace
table spaces represented by pg_tablespace
These tables are global to each PostgreSQL cluster.
Keys are names (strings) and must be unique within cluster.
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Representing Databases (cont)

pg_database contains information about databases:
oid, datname, datdba, datacl[], encoding,
pg_namespace contains information about schemata:
oid, nspname, nspowner, nspacl[]
pg_tablespace contains information about tablespaces:
oid, spcname, spcowner, spcacl[]
PostgreSQL represents access via array of access items:

Role=Privileges/Grantor
where Privileges is a string enumerating privileges, e.g.

jas=arwdRxt/jas,fred=r/jas,joe=rwad/jas

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Representing Tables

Representing one table needs tuples in several catalog tables.
Due to O-O heritage, base table for tables is called pg_class.
The pg_class table also handles other table-like objects:
views represents attributes/domains of view
composite (tuple) types from CREATE TYPE AS
sequences, indexes (top-level defn), other special objects
All tuples in pg_class have an OID, used as primary key.
Some fields from the pg_class table:
oid, relname, relnamespace, reltype, relowner
relkind, reltuples, relnatts, relhaspkey, relacl,
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<< Representing Tables (cont)Details of catalog tables representing database tables pg_class holds core information about tables relname, relnamespace, reltype, relowner, … relkind, relnatts, relhaspkey, relacl[], … pg_attribute contains information about attributesattrelid, attname, atttypid, attnum, … pg_type contains information about typestypname, typnamespace, typowner, typlen, … typtype, typrelid, typinput, typoutput, … COMP9315 21T1 Catalogs [11/11]Produced: 15 Feb 2021