Re: [proposal] de-TOAST'ing using a iterator

init_toast_buffer():

+ * Note the constrain buf->position <= buf->limit may be broken
+ * at initialization. Make sure that the constrain is satisfied
+ * when consume chars.

s/constrain/constraint/ (2 times)
s/consume/consuming/

Also, this comment might be better at the top the whole function?

The check
if (iter->buf != iter->fetch_datum_iterator->buf)
is what we need to do for the compressed case. Could we use this
directly instead of having a separate state variable iter->compressed,
with a macro like this?
#define TOAST_ITER_COMPRESSED(iter) \
    (iter->buf != iter->fetch_datum_iterator->buf)

+ * If "ctrlc" field in iterator is equal to INVALID_CTRLC, it means that
+ * the field is invalid and need to read the control byte from the
+ * source buffer in the next iteration, see pglz_decompress_iterate().
+ */
+#define INVALID_CTRLC 8

I think the macro might be better placed in pg_lzcompress.h, and for
consistency used in pglz_decompress(). Then the comment can be shorter
and more general. With my additional comment in
init_detoast_iterator(), hopefully it will be clear to readers.