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Daniel Westermann

Can I do it with PostgreSQL? – 5 – Generating DDL commands

From time to time it is very useful that you can generate the DDL commands for existing objects (Tables, Indexes, whole Schema …). In Oracle you can either use the dbms_metadata PL/SQL package for this or use expdp/impdp to generate the statements out of a dump file. What options do you have in PostgreSQL? Note: We’ll not look at any third party tools you could use for that, only plain PostgreSQL.

As always we’ll need some objects to test with, so here we go:

\c postgres
drop database if exists ddl;
create database ddl;
\c ddl
create table t1 ( a int, b int );
insert into t1 (a,b)
       values (generate_series(1,1000000)
              ,generate_series(1,1000000));
select count(*) from t1;
create index i1 on t1(a);
create unique index i2 on t1(b);
create view v1 as select a from t1;
alter table t1 add constraint con1 check ( a < 2000000 );
\d t1
CREATE FUNCTION add(integer, integer) RETURNS integer
    AS 'select $1 + $2;'
    LANGUAGE SQL
    IMMUTABLE
    RETURNS NULL ON NULL INPUT;

PostgreSQL comes with a set of administration functions which can be used to query various stuff. Some are there to get the definitions for your objects.

You can get the definition of a view:

([email protected][local]:5439) [ddl] > select pg_get_viewdef('v1'::regclass, true);
 pg_get_viewdef 
----------------
  SELECT t1.a  +
    FROM t1;
(1 row)

You can get the definition of a constraint:

([email protected][local]:5439) [ddl] > SELECT conname
                                     , pg_get_constraintdef(r.oid, true) as definition
                                  FROM pg_constraint r
                                 WHERE r.conrelid = 't1'::regclass;
 conname |     definition      
---------+---------------------
 con1    | CHECK (a < 2000000)

You can get the definition of a function:

([email protected][local]:5439) [ddl] > SELECT proname
     , pg_get_functiondef(a.oid)
  FROM pg_proc a
 WHERE a.proname = 'add';
 proname |                   pg_get_functiondef                    
---------+---------------------------------------------------------
 add     | CREATE OR REPLACE FUNCTION public.add(integer, integer)+
         |  RETURNS integer                                       +
         |  LANGUAGE sql                                          +
         |  IMMUTABLE STRICT                                      +
         | AS $function$select $1 + $2;$function$                 +
         | 
--OR
([email protected][local]:5439) [ddl] > SELECT pg_get_functiondef(to_regproc('add'));
                   pg_get_functiondef                    
---------------------------------------------------------
 CREATE OR REPLACE FUNCTION public.add(integer, integer)+
  RETURNS integer                                       +
  LANGUAGE sql                                          +
  IMMUTABLE STRICT                                      +
 AS $function$select $1 + $2;$function$                 +

You can get the definition of an index:

([email protected][local]:5439) [ddl] > select pg_get_indexdef('i1'::regclass);
            pg_get_indexdef            
---------------------------------------
 CREATE INDEX i1 ON t1 USING btree (a)
(1 row)

But surprisingly you can not get the DDL for a table. There is just no function available to do this. How can you do that without concatenating the definitions you can get out of the PostgreSQL catalog? The only option I am aware of is pg_dump:

[email protected]:/home/postgres/ [PG961] pg_dump -s -t t1 ddl | egrep -v "^--|^$"
SET statement_timeout = 0;
SET lock_timeout = 0;
SET idle_in_transaction_session_timeout = 0;
SET client_encoding = 'UTF8';
SET standard_conforming_strings = on;
SET check_function_bodies = false;
SET client_min_messages = warning;
SET row_security = off;
SET search_path = public, pg_catalog;
SET default_tablespace = '';
SET default_with_oids = false;
CREATE TABLE t1 (
    a integer,
    b integer,
    CONSTRAINT con1 CHECK ((a < 2000000))
);
ALTER TABLE t1 OWNER TO postgres;
CREATE INDEX i1 ON t1 USING btree (a);
CREATE UNIQUE INDEX i2 ON t1 USING btree (b);

Using the “-s” (schema only) and “-t” (tables) options you get the DDL for the complete table. Not as handy as in Oracle where you can do this in sqlplus but it works and produces a result you can work with.

Of course you can always create the DDLs for your own by querying the catalog, e.g. pg_attribute which holds all the column definitions for the tables:

    Table "pg_catalog.pg_attribute"
    Column     |   Type    | Modifiers 
---------------+-----------+-----------
 attrelid      | oid       | not null
 attname       | name      | not null
 atttypid      | oid       | not null
 attstattarget | integer   | not null
 attlen        | smallint  | not null
 attnum        | smallint  | not null
 attndims      | integer   | not null
 attcacheoff   | integer   | not null
 atttypmod     | integer   | not null
 attbyval      | boolean   | not null
 attstorage    | "char"    | not null
 attalign      | "char"    | not null
 attnotnull    | boolean   | not null
 atthasdef     | boolean   | not null
 attisdropped  | boolean   | not null
 attislocal    | boolean   | not null
 attinhcount   | integer   | not null
 attcollation  | oid       | not null
 attacl        | aclitem[] | 
 attoptions    | text[]    | 
 attfdwoptions | text[]    | 
Indexes:
    "pg_attribute_relid_attnam_index" UNIQUE, btree (attrelid, attname)
    "pg_attribute_relid_attnum_index" UNIQUE, btree (attrelid, attnum)

One nasty way which which is even documented on the PostgreSQL wiki is this:

([email protected][local]:5439) [ddl] > create extension plperlu;
CREATE EXTENSION
Time: 90.074 ms
([email protected][local]:5439) [ddl] > \dx
                      List of installed extensions
  Name   | Version |   Schema   |              Description               
---------+---------+------------+----------------------------------------
 plperlu | 1.0     | pg_catalog | PL/PerlU untrusted procedural language
 plpgsql | 1.0     | pg_catalog | PL/pgSQL procedural language

([email protected][local]:5439) [ddl] > CREATE OR REPLACE FUNCTION system(text) RETURNS text 
AS 'my $cmd=shift; return `cd /tmp;$cmd`;' LANGUAGE plperlu;
CREATE FUNCTION

([email protected][local]:5439) [ddl] > select system('pg_dump -s -t t1 ddl | egrep -v "^--|^$"');
                    system                     
-----------------------------------------------
 SET statement_timeout = 0;                   +
 SET lock_timeout = 0;                        +
 SET idle_in_transaction_session_timeout = 0; +
 SET client_encoding = 'UTF8';                +
 SET standard_conforming_strings = on;        +
 SET check_function_bodies = false;           +
 SET client_min_messages = warning;           +
 SET row_security = off;                      +
 SET search_path = public, pg_catalog;        +
 SET default_tablespace = '';                 +
 SET default_with_oids = false;               +
 CREATE TABLE t1 (                            +
     a integer,                               +
     b integer,                               +
     CONSTRAINT con1 CHECK ((a < 2000000))    +
 );                                           +
 ALTER TABLE t1 OWNER TO postgres;            +
 CREATE INDEX i1 ON t1 USING btree (a);       +
 CREATE UNIQUE INDEX i2 ON t1 USING btree (b);+
 

Can be a workaround. Hope this helps…

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Daniel Westermann
Daniel Westermann

Principal Consultant & Technology Leader Open Infrastructure