How to duplicate schemas in PostgreSQL

Question

I have a database with schema public and schema_A. I need to create a new schema schema_b with the same structure than schema_a

Answer 1

A bit late to the party but, some sql here could help you along your way:

get schema oid:

namespace_id = SELECT oid 
                  FROM pg_namespace 
                 WHERE nspname = '';

get table's oid:

table_id = SELECT relfilenode 
                FROM pg_class 
               WHERE relnamespace = '' AND relname = ''

get foreign key constraints:

SELECT con.conname, pg_catalog.pg_get_constraintdef(con.oid) AS condef 
  FROM pg_catalog.pg_constraint AS con 
  JOIN pg_class AS cl ON cl.relnamespace = con.connamespace AND cl.relfilenode = con.conrelid 
 WHERE con.conrelid = ''::pg_catalog.oid AND con.contype = 'f';

A good resource for PostgreSQL system tables can be found here. Additionally, you can learn more about the internal queries pg_dump makes to gather dump information by viewing it's source code.

Probably the easiest way to see how pg_dump gathers all your data would be to use strace on it, like so:

$ strace -f -e sendto -s8192 -o pg_dump.trace pg_dump -s -n 
$ grep -oP '(SET|SELECT)\s.+(?=\\0)' pg_dump.trace

You'll still have to sort through the morass of statements but, it should help you piece together a cloning tool programmatically and avoid having to drop to a shell to invoke pg_dump.