The system catalogs are the place where a relational database management system stores schema metadata, such as information about tables and columns, and internal bookkeeping information. PostgreSQL's system catalogs are regular tables. You can drop and recreate the tables, add columns, insert and update values, and severely mess up your system that way. Normally one should not change the system catalogs by hand, there are always SQL commands to do that. (For example, CREATE DATABASE inserts a row into the pg_database catalog -- and actually creates the database on disk.) There are some exceptions for especially esoteric operations, such as adding index access methods.
Most system catalogs are copied from the template database during database creation, and are thereafter database-specific. A few catalogs are physically shared across all databases in an installation; these are marked in the descriptions of the individual catalogs.
Table 3-1. System Catalogs
|pg_am||index access methods|
|pg_amop||access method operators|
|pg_amproc||access method support procedures|
|pg_attrdef||column default values|
|pg_attribute||table columns ("attributes", "fields")|
|pg_cast||casts (data type conversions)|
|pg_class||tables, indexes, sequences ("relations")|
|pg_constraint||check constraints, unique / primary key constraints, foreign key constraints|
|pg_conversion||encoding conversion information|
|pg_database||databases within this database cluster|
|pg_depend||dependencies between database objects|
|pg_description||descriptions or comments on database objects|
|pg_group||groups of database users|
|pg_index||additional index information|
|pg_inherits||table inheritance hierarchy|
|pg_language||languages for writing functions|
|pg_opclass||index access method operator classes|
|pg_proc||functions and procedures|
|pg_rewrite||query rewriter rules|
More detailed documentation of each catalog follows below.
Here is s simple solution for finding the size of varchar's type column:
SELECT pg_class.relname, pg_attribute.attname, pg_type.typname,
FROM pg_class, pg_attribute, pg_type
WHERE pg_attribute.attrelid = pg_class.oid
AND pg_attribute.atttypid = pg_type.oid
AND pg_class.relname = 'TABLE_NAME'
AND pg_attribute.attname = 'COLUMN_NAME'
TABLE_NAME | COLUMN_NAME | varchar | SIZE
Remeber that you have to decrease the value of the filed pg_attribute.atttypmod by 4, because all variable-length types begin with a length field of exactly 4 bytes.