How to create sequence if not exists

Postgres 9.5 or later

IF NOT EXISTS was added to CREATE SEQUENCE in Postgres 9.5. That’s the simple solution now:

CREATE SEQUENCE IF NOT EXISTS myschema.myseq;

But consider details of the outdated answer anyway …
And you know about serial or IDENTITY columns, right?

• Auto increment table column

Postgres 9.4 or older

Sequences share the namespace with several other table-like objects. The manual:

The sequence name must be distinct from the name of any other
sequence, table, index, view, or foreign table in the same schema.

Bold emphasis mine. So there are three cases:

1. Name does not exist. -> Create sequence.
2. Sequence with the same name exists. -> Do nothing? Any output? Any logging?
3. Other conflicting object with the same name exists. -> Do something? Any output? Any logging?

Specify what to do in either case. A DO statement could look like this:

DO
$do$
DECLARE
   _kind "char";
BEGIN
   SELECT relkind
   FROM   pg_class
   WHERE  oid = 'myschema.myseq'::regclass  -- sequence name, optionally schema-qualified
   INTO  _kind;

   IF NOT FOUND THEN       -- name is free
      CREATE SEQUENCE myschema.myseq;
   ELSIF _kind = 'S' THEN  -- sequence exists
      -- do nothing?
   ELSE                    -- object name exists for different kind
      -- do something!
   END IF;
END
$do$;

Object types (relkind) in pg_class according to the manual:

r = ordinary table
i = index
S = sequence
v = view
m = materialized view
c = composite type
t = TOAST table
f = foreign table

Related:

• How to check if a table exists in a given schema