[SOLVED] CS计算机代考程序代写 SQL database >>

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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 attributes•attrelid, attname, atttypid, attnum, … pg_type contains information about types•typname, typnamespace, typowner, typlen, … •typtype, typrelid, typinput, typoutput, … COMP9315 21T1 ♢ Catalogs ♢ [11/11]Produced: 15 Feb 2021