Trying to convert simple execution stored procedure to postgres function - can't get CURRVAL('table_seq') to function

Question

The MySQL Stored Procedure was:

     BEGIN
     set @sql=_sql;
     PREPARE stmt FROM @sql; 
     EXECUTE stmt;
     DEALLOCATE PREPARE stmt;
     set _ires=LAST_INSERT_ID();
     END$$

I tried to convert it to:

     BEGIN  
     EXECUTE _sql;
     SELECT INTO _ires CURRVAL('table_seq');
     RETURN;
     END;

I get the error:

SQL error:

ERROR:  relation "table_seq" does not exist
LINE 1: SELECT CURRVAL('table_seq')
                       ^
QUERY:  SELECT CURRVAL('table_seq')
CONTEXT:  PL/pgSQL function "myexecins" line 4 at SQL statement
In statement:
SELECT myexecins('SELECT * FROM tblbilldate WHERE billid = 2')

The query used is for testing purposes only. I believe this function is used to get the row id of the inserted or created row from the query. Any Suggestions?

Answer 1

When you create tables with serial columns, sequences are, by default, named as tablename_columnname_seq, but it seems that you're trying to access tablename_seq. So with a table called foobar and primary key column foobar_id it ends up being foobar_foobar_id_seq.

By the way, a cleaner way to get the primary key after an insert is using the RETURNING clause in INSERT. E.g.:

_sql = 'INSERT INTO sometable (foobar) VALUES (123) RETURNING sometable_id';
EXECUTE _sql INTO _ires;

Answer 2

PostgreSQL is saying that there is no sequence called "table_seq". Are you sure that that is the right name? The name you would use would depend on what is in _sql as each SERIAL or BIGSERIAL gets its own sequence, you can also define sequences and wire them up by hand.

In any case, lastval() is a closer match to MySQL's LAST_INSERT_ID(), lastval() returns the most recently returned value from any sequence in the current session:

lastval
Return the value most recently returned by nextval in the current session. This function is identical to currval, except that instead of taking the sequence name as an argument it fetches the value of the last sequence used by nextval in the current session. It is an error to call lastval if nextval has not yet been called in the current session.

Using lastval() also means that you don't have to worry about what's in _sql, unless of course it doesn't use a sequence at all.