How do I use currval() in PostgreSQL to get the last inserted id?

Question

I have a table:

CREATE TABLE names (id serial, name varchar(20))

I want the "last inserted id" from that table, without using RETURNING id on insert. There seem to be a function CURRVAL(), but I don't understand how to use it.

I have tried with:

SELECT CURRVAL() AS id FROM names_id_seq
SELECT CURRVAL('names_id_seq')
SELECT CURRVAL('names_id_seq'::regclass)

but none of them work. How can I use currval() to get the last inserted id?

Answer 1

If you create a column as serial PostgreSQL automatically creates a sequence for that.

The name of the sequence is autogenerated and is always tablename_columnname_seq, in your case the sequence will be names names_id_seq.

After inserting into the table, you can call currval() with that sequence name:

postgres=> CREATE TABLE names in schema_name (id serial, name varchar(20));
CREATE TABLE
postgres=> insert into names (name) values ('Arthur Dent');
INSERT 0 1
postgres=> select currval('names_id_seq');
 currval
---------
       1
(1 row)
postgres=>

Answer 2

You need to call nextval for this sequence in this session before currval:

create sequence serial;
select nextval('serial');
 nextval
---------
       1
(1 row)

select currval('serial');
 currval
---------
       1
(1 row)

so you cannot find the 'last inserted id' from the sequence unless the insert is done in the same session (a transaction might roll back but the sequence will not)

as pointed out in a_horse's answer, create table with a column of type serial will automatically create a sequence and use it to generate the default value for the column, so an insert normally accesses nextval implicitly:

create table my_table(id serial);
NOTICE:  CREATE TABLE will create implicit sequence "my_table_id_seq" for 
         serial column "my_table.id"

\d my_table
                          Table "stack.my_table"
 Column |  Type   |                       Modifiers
--------+---------+-------------------------------------------------------
 id     | integer | not null default nextval('my_table_id_seq'::regclass)

insert into my_table default values;
select currval('my_table_id_seq');
 currval
---------
       1
(1 row)