On 12th of February, Robert Haas committed patch:
Teach ALTER TABLE .. SET DATA TYPE TO avoid SOME TABLE rewrites. WHEN the OLD TYPE IS BINARY coercible TO the NEW TYPE AND the USING clause does NOT CHANGE the COLUMN contents, we can avoid a FULL TABLE rewrite, though any indexes ON the affected COLUMNS will still need TO be rebuilt. This applies, FOR example, WHEN changing a VARCHAR COLUMN TO be OF TYPE text. The prior coding assumed that the SET OF operations that force a rewrite IS identical TO the SET OF operations that must be propagated TO TABLES making USE OF the affected TABLE's rowtype. This is no longer true: even though the tuples in those tables wouldn't need TO be modified, the DATA TYPE CHANGE invalidate indexes built USING those composite TYPE COLUMNS. Indexes ON the TABLE we're actually modifying can be invalidated too, of course, but the existing machinery is sufficient to handle that case. Along the way, add some debugging messages that make it possible to understand what operations ALTER TABLE is actually performing in these cases. Noah Misch and Robert Haas
Later on, on 15th, he committed second patch with few more cases where rewrite can be avoided.
Continue reading Waiting for 9.1 – Rewrite-less changing types of column