Change column datatype without rewrite when data remains unchanged

Question

For example:

• timestamp(tz) to int8
• date to int4
• json to text

The raw data is unchanged, and the only difference is how that data is handled. So it should be possible to change back and forth cleanly and instantly.

1. How do I check if this is actually the case?

2. If it is not the case (table gets rewritten), how do I make it do it without rewriting?

Answer 1

Like the documentation says:

[...] changing the type of an existing column will require the entire table and its indexes to be rewritten. As an exception, when changing the type of an existing column, if the USING clause does not change the column contents and the old type is either binary coercible to the new type or an unconstrained domain over the new type, a table rewrite is not needed; but any indexes on the affected columns must still be rebuilt.

So the key is that the type conversion is binary coercible, in other words, you need a type cast between these types that is defined WITHOUT FUNCTION.

This is not the case for the data types you mention in your question, but if you are a superuser, you can create such a cast and then use it with ALTER TABLE:

CREATE CAST (date AS int4) WITHOUT FUNCTION;
ALTER TABLE mytable ALTER datecol TYPE int4 USING (datecol::int4);
DROP CAST (date AS int4);

Of course you have to understand the internal representation of date for that to make sense.

It is a good idea to remove the cast after you are done, because adding type casts to PostgreSQL can easily lead to ambiguities during type resolution, which will cause errors.