On 01/20/19 12:48, Pavel Stehule wrote: >> >> Accordingly, I think the paragraph beginning "Unlike regular PostgreSQL >> functions," is more likely to perplex readers than to enlighten them. >> What it says about column_expression does not seem to lead to any useful >> difference from the behavior if it were "just like regular PostgreSQL >> functions". > > regular Postgres' functions has evaluated all arguments before own > execution. I think so this note is related much more to expressions used as > defaults.
Sure, but again, is there an example, or can one easily be constructed, that shows the default expressions working in such a way? I am not able to use a default expression to refer to an earlier column in the column list of the xmltable call. I am able to use a default expression to refer to a column of an earlier FROM item in the enclosing SELECT. But such a query ends up having LATERAL form (whether or not the word LATERAL is used), and re-executes xmltable whenever the referenced column value changes. In that case, whether the default argument is evaluated at function entry or later doesn't seem to matter: the function is re-executed, so evaluating the new default at the time of entry is sufficient. So, I have still not been able to construct a query that requires the deferred evaluation behavior. But perhaps there is a way I haven't thought of. Regards, -Chap
