In plpgsql, how can I create and execute a command-string using on a variable number of identifiers?

Question

In postgres 9.1 I'd like to create a function that takes an index name, a table name and a variable number of columns, constructs an index, and then does some other things.

My current approach is to use plpgsql and construct a dynamic command to execute. However, I'm getting tripped up when trying to use quote_ident to protect all of the identifiers. Code thus far:

CREATE OR REPLACE FUNCTION my_create_index(indexname text, tablename text, VARIADIC arr text[]) RETURNS void AS $$
  DECLARE
    command_string text;
  BEGIN
    command_string := 'CREATE INDEX ' || quote_ident(indexname) || ' ON ' ||
         quote_ident(tablename) || ' (' ||
         format(repeat('%I ', array_length($3, 1)), VARIADIC $3) ||
         ')';

    -- display the string
    RAISE NOTICE '%', command_string;

    -- execute the string
    EXECUTE command_string;

    -- (do other stuff)
  END;
$$ LANGUAGE plpgsql;

The code appears to be successful when passing one column name, but with two or more I get the following error:

ERROR:  too few arguments for format

What am I doing wrong? (Presumably something with format or my use of VARIADIC.)

Thank you!

Answer 1

You don't generally to use both format and quote_ident. format is smart enough to do identifer quoting its self; you show that yourself in part of your expression, while using unnecessary concatenation and quote_ident calls elsewhere.

However, I'm seeing the same issue you are with calling format with a VARIADIC array argument, and I suspect you've found a bug.

Here's a workaround until I spot what's going on:

command_string := format('CREATE INDEX %I ON %I (%s)', 
    indexname, tablename, (
      SELECT string_agg(quote_ident(x), ', ') 
      FROM unnest($3) x
    )
);

Note that in your original code, '%I ' should've been '%I, '.

Yes, bug confirmed, and it's been reported before but looks like it never got checked up on. The same bug exists in concat and concat_ws. These functions fail to check for the VARIADIC argument flag.

Observe:

regress=> SELECT format('%I', VARIADIC ARRAY['b','c','d']);
  format   
-----------
 "{b,c,d}"
(1 row)

regress=> SELECT format('%I', 'b','c','d');
 format 
--------
 b
(1 row)

regress=> SELECT format('%I, %I, %I', VARIADIC ARRAY['b','c','d']);
ERROR:  too few arguments for format

The only workaround I can think of is either avoiding them (as shown above) or using EXECUTE to construct the function call dynamically. In the case of format that's totally redundant, so I'd just use the above sub-query with unnest.

I'll try to chase this up on the PostgreSQL mailing lists, but I'm going to be insanely busy next week, so poke me or post on pgsql-bugs with a link back to this question yourself if you don't see a follow-up by mid week-after-next.

Answer 2

You cannot use a VARIADIC parameter in format function now :(. It is a known bug, that will be fixed in PostgreSQL 9.3 (I hope). You can try use patch http://archives.postgresql.org/message-id/CAFj8pRDZc7wG1ewnmqUWkjDsB78Pu=Oj_WZ8_CS7qJUCKRUNmQ@mail.gmail.com