Re: Should we increase the default vacuum_cost_limit?

Andrew Dunstan <andrew(dot)dunstan(at)2ndquadrant(dot)com> writes:
> On 3/9/19 4:28 AM, David Rowley wrote:
>> I agree that vacuum_cost_delay might not be granular enough, however.
>> If we're going to change the vacuum_cost_delay into microseconds, then
>> I'm a little concerned that it'll silently break existing code that
>> sets it. Scripts that do manual off-peak vacuums are pretty common
>> out in the wild.

> Maybe we could leave the default units as msec but store it and allow
> specifying as usec. Not sure how well the GUC mechanism would cope with
> that.

I took a quick look at that and I'm afraid it'd be a mess. GUC doesn't
really distinguish between a variable's storage unit, its default input
unit, or its default output unit (as seen in e.g. pg_settings). Perhaps
we could split those into two or three distinct concepts, but it seems
complicated and bug-prone. Also I think we'd still be forced into
making obviously-incompatible changes in what pg_settings shows for
this variable, since what it shows right now is integer ms. That last
isn't a deal-breaker perhaps, but 100% compatibility isn't going to
happen this way.

The idea of converting vacuum_cost_delay into a float variable, while
keeping its native unit as ms, seems probably more feasible from a
compatibility standpoint. There are two sub-possibilities:

1. Just do that and lose units support for the variable. I don't
think this is totally unreasonable, because up to now ms is the
*only* workable unit for it:

regression=# set vacuum_cost_delay = '1s';
ERROR: 1000 is outside the valid range for parameter "vacuum_cost_delay" (0 .. 100)

Still, it'd mean that anyone who'd been explicitly setting it with
an "ms" qualifier would have to change their postgresql.conf entry.

2. Add support for units for float variables, too. I don't think
this'd be a huge amount of work, and we'd surely have other uses
for it in the long run.

I'm inclined to go look into #2. Anybody think this is a bad idea?

regards, tom lane