Re: Large expressions in indexes can't be stored (non-TOASTable)

On Mon, Sep 23, 2024 at 04:00:00PM +0300, Alexander Lakhin wrote:
> I tested it with two code modifications (1st is to make each created
> expression index TOASTed (by prepending 1M of spaces to the indexeprs
> value) and 2nd to make each created index an expression index (by
> modifying index_elem_options in gram.y) - both modifications are kludgy so
> I don't dare to publish them) and found no other snapshot-related issues
> during `make check-world`.

Thanks. Here is an updated patch with tests and comments. I've also moved
the calls to PushActiveSnapshot()/PopActiveSnapshot() to surround only the
section of code where the snapshot is needed. Besides being more similar
in style to other fixes I found, I think this is safer because much of this
code is cautious to avoid deadlocks. For example, DefineIndex() has the
following comment:

/*
* The snapshot subsystem could still contain registered snapshots that
* are holding back our process's advertised xmin; in particular, if
* default_transaction_isolation = serializable, there is a transaction
* snapshot that is still active. The CatalogSnapshot is likewise a
* hazard. To ensure no deadlocks, we must commit and start yet another
* transaction, and do our wait before any snapshot has been taken in it.
*/

I carefully inspected all the code paths this patch touches, and I think
I've got all the details right, but I would be grateful if someone else
could take a look.

--
nathan