How to describe view in postgresql

In every RDBMS database there are default dictionary read only tables to contain the metadata of complete database. These data dictionary views helps to find out database default structure and application objects information.

In PostgreSQL database also there are user defined objects as well as default tables and views which got created during the db cluster installation. 

to use those views we need to now the columns of tables/ data dictionary views.

Example : To verify database information there is view called pg_database.

\d  <dictionary view>   describe view in postgresql

postgres=# \d pg_database;

               Table "pg_catalog.pg_database"

    Column     |   Type    | Collation | Nullable | Default

---------------+-----------+-----------+----------+---------

 oid           | oid       |           | not null |

 datname       | name      |           | not null |

 datdba        | oid       |           | not null |

 encoding      | integer   |           | not null |

 datcollate    | name      |           | not null |

 datctype      | name      |           | not null |

 datistemplate | boolean   |           | not null |

 datallowconn  | boolean   |           | not null |

 datconnlimit  | integer   |           | not null |

 datlastsysoid | oid       |           | not null |

 datfrozenxid  | xid       |           | not null |

 datminmxid    | xid       |           | not null |

 dattablespace | oid       |           | not null |

 datacl        | aclitem[] |           |          |

Indexes:

    "pg_database_oid_index" PRIMARY KEY, btree (oid), tablespace "pg_global"

    "pg_database_datname_index" UNIQUE CONSTRAINT, btree (datname), tablespace "pg_global"

Tablespace: "pg_global"

postgres=# \d pg_tablespace;

            Table "pg_catalog.pg_tablespace"

   Column   |   Type    | Collation | Nullable | Default

------------+-----------+-----------+----------+---------

 oid        | oid       |           | not null |

 spcname    | name      |           | not null |

 spcowner   | oid       |           | not null |

 spcacl     | aclitem[] |           |          |

 spcoptions | text[]    | C         |          |

Indexes:

    "pg_tablespace_oid_index" PRIMARY KEY, btree (oid), tablespace "pg_global"

    "pg_tablespace_spcname_index" UNIQUE CONSTRAINT, btree (spcname), tablespace "pg_global"

Tablespace: "pg_global"