Re: row filtering for logical replication

On Sun, Dec 26, 2021, at 1:09 PM, Alvaro Herrera wrote:
> On 2021-Dec-26, Euler Taveira wrote:
>
> > On Sat, Dec 25, 2021, at 1:20 AM, Amit Kapila wrote:
> > > On Fri, Dec 24, 2021 at 11:04 AM Peter Smith <smithpb2250(at)gmail(dot)com> wrote:
> > > >
> > > > So, IMO the PG docs wording for this part should be relaxed a bit.
> > > >
> > > > e.g.
> > > > BEFORE:
> > > > + A nullable column in the <literal>WHERE</literal> clause could cause the
> > > > + expression to evaluate to false; avoid using columns without not-null
> > > > + constraints in the <literal>WHERE</literal> clause.
> > > > AFTER:
> > > > + A nullable column in the <literal>WHERE</literal> clause could cause the
> > > > + expression to evaluate to false. To avoid unexpected results, any possible
> > > > + null values should be accounted for.
>
> Is this actually correct? I think a null value would cause the
> expression to evaluate to null, not false; the issue is that the filter
> considers a null value as not matching (right?). Maybe it's better to
> spell that out explicitly; both these wordings seem distracting.
[Reading it again...] I think it is referring to the
pgoutput_row_filter_exec_expr() return. That's not accurate because it is
talking about the expression and the expression returns true, false and null.
However, the referred function returns only true or false. I agree that we
should explictily mention that a null return means the row won't be published.

> You have this elsewhere:
>
> + If the optional <literal>WHERE</literal> clause is specified, only rows
> + that satisfy the <replaceable class="parameter">expression</replaceable>
> + will be published. Note that parentheses are required around the
> + expression. It has no effect on <literal>TRUNCATE</literal> commands.
>
> Maybe this whole thing is clearer if you just say "If the optional WHERE
> clause is specified, rows for which the expression returns false or null
> will not be published." With that it should be fairly clear what
> happens if you have NULL values in the columns used in the expression,
> and you can just delete that phrase you're discussing.
Your proposal sounds good to me.

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