Re: [Bizgres-general] WAL bypass for INSERT, UPDATE and

On Thu, 2006-01-05 at 12:27 -0500, Bruce Momjian wrote:

> Seems like a nice optimization.

Negative thoughts: Toast tables have a toast index on them, yes? We have
agreed that we cannot use the optimization if we have indexes on the
main table. It follows that we cannot use the optimization if we have
*any* toasted data, since that would require a pointer between two
blocks, which would not be correctly recovered following a crash. If we
log the toast table then there could be a mismatch between heap and
toast table; if we don't log the toast table there could be a mismatch
between toast table and toast index.

We can test to see if the toast table is empty when we do ALTER TABLE,
but loading operations may try to create toasted data rows.

Presumably that means we must either:
i) abort a COPY if we get a toastable value
ii) if we get a toastable value, insert the row into a new block, which
we do logging of, then also log the toast insert and the toast index
insert - i.e. some blocks we log, others not

This is still useful for many applications, IMHO, but the list of
restrictions seems to be growing. Worse, we wouldn't know that the toast
tables were empty until after we did the COPY TO for a pg_dump, so we
wouldn't be able to retrospectively add an ALTER TABLE command ahead of
the COPY.

Thoughts? Hopefully there are some flaws in my thinking here,

Best Regards, Simon Riggs