Re: row filtering for logical replication

On Tue, Aug 24, 2021, at 4:46 AM, Peter Smith wrote:
> I have used debug logging to confirm that what Amit wrote [1] is
> correct; the row-filter ExprState of *every* table's row_filter will
> be invalidated (and so subsequently gets rebuilt) when the user
> changes the PUBLICATION tables. This was a side-effect of the
> rel_sync_cache_publication_cb which is freeing the cached ExprState
> and setting the entry->replicate_valid = false; for *every* entry.
>
> So yes, the ExprCache is getting rebuilt for some situations where it
> is not strictly necessary to do so.
I'm afraid we are overenginnering this feature. We already have a cache
mechanism that was suggested (that shows a small improvement). As you said the
gain for this new improvement is zero or minimal (it depends on your logical
replication setup/maintenance).

> 1. Although the ExprState cache is effective, in practice the
> performance improvement was not very much. My previous results [2]
> showed only about 2sec saving for 100K calls to the
> pgoutput_row_filter function. So I think eliminating just one or two
> unnecessary calls in the get_rel_sync_entry is going to make zero
> observable difference.
>
> 2. IMO it is safe to expect that the ALTER PUBLICATION is a rare
> operation relative to the number of times that pgoutput_row_filter
> will be called (the pgoutput_row_filter is quite a "hot" function
> since it is called for every INSERT/UPDATE/DELETE). It will be orders
> of magnitude difference 1:1000, 1:100000 etc.
>
> ~~
>
> Anyway, I have implemented the suggested cache change because I agree
> it is probably theoretically superior, even if in practice there is
> almost no difference.
I didn't inspect your patch carefully but it seems you add another List to
control this new cache mechanism. I don't like it. IMO if we can use the data
structures that we have now, let's implement your idea; otherwise, -1 for this
new micro optimization.

[By the way, it took some time to extract what you changed. Since we're trading
patches, I personally appreciate if you can send a patch on the top of the
current one. I have some changes too and it is time consuming incorporating
changes in the main patch.]

--
Euler Taveira
EDB https://www.enterprisedb.com/