The data types of the columns in the temporary view are defined by those of the initial subquery. The data types of the columns from the recursive subquery must match. The database server automatically attempts to convert the values returned by the recursive subquery to match those of the initial query. If this is not possible, or if information may be lost in the conversion, an error is generated.

In general, explicit casts are often required when the initial subquery returns a literal value or NULL. Explicit casts may also be required when the initial subquery selects values from different columns than the recursive subquery.

Casts may be required if the columns of the initial subquery do not have the same domains as those of the recursive subquery. Casts must always be applied to NULL values in the initial subquery.

For example, the bookshelf parts explosion sample works correctly because the initial subquery returns rows from the bookcase table, and inherits the data types of the selected columns. See Parts explosion problems.

If this query is rewritten as follows, explicit casts are required.

WITH RECURSIVE parts ( component, subcomponent, quantity ) AS
( SELECT NULL, 'bookcase', 1         -- ERROR! Wrong domains!
    UNION ALL
  SELECT b.component, b.subcomponent,
         p.quantity * b.quantity
  FROM parts p JOIN bookcase b
    ON p.subcomponent = b.component )
SELECT * FROM parts
ORDER BY component, subcomponent;

Without casting, errors result for the following reasons:

No cast is required for the second column because this column of the initial query is already a string.

Casting the data types in the initial subquery allows the query to behave as intended:

WITH RECURSIVE parts ( component, subcomponent, quantity ) AS
( SELECT CAST( NULL AS VARCHAR ),  -- CASTs must be used
         'bookcase',               -- to declare the
         CAST( 1   AS INT )        -- correct datatypes
      UNION ALL
  SELECT b.component, b.subcomponent,
         p.quantity * b.quantity
  FROM parts p JOIN bookcase b
    ON p.subcomponent = b.component )
SELECT * FROM parts
ORDER BY component, subcomponent;