## 10.6. `SELECT` Output Columns

The rules given in the preceding sections will result in assignment of non-`unknown` data types to all expressions in a SQL query, except for unspecified-type literals that appear as simple output columns of a `SELECT` command. For example, in

    SELECT 'Hello World';

there is nothing to identify what type the string literal should be taken as. In this situation Postgres Pro will fall back to resolving the literal's type as `text`.

When the `SELECT` is one arm of a `UNION` (or `INTERSECT` or `EXCEPT`) construct, or when it appears within `INSERT ... SELECT`, this rule is not applied since rules given in preceding sections take precedence. The type of an unspecified-type literal can be taken from the other `UNION` arm in the first case, or from the destination column in the second case.

`RETURNING` lists are treated the same as `SELECT` output lists for this purpose.

### Note

Prior to Postgres Pro 10, this rule did not exist, and unspecified-type literals in a `SELECT` output list were left as type `unknown`. That had assorted bad consequences, so it's been changed.