Re: On disable_cost

On 8/23/24 5:33 PM, Jonathan S. Katz wrote:
> On 8/23/24 3:32 PM, Tom Lane wrote:
>> Robert Haas <robertmhaas(at)gmail(dot)com> writes:

>> I think it's not that bad, because we can limit the knowledge of this
>> hack to the amcostestimate interface, which doesn't really deal in
>> "the user told us not to do it this way" at all.  That argues against
>> my first proposal though (having amcostestimate touch disabled_nodes
>> directly).  I now think that a reasonable compromise is to say that
>> setting indexTotalCost to +Inf signals that "the AM cannot support
>> it".  That's not conflated too much with the other case, since even a
>> crazy-expensive cost estimate surely ought to be finite.  We can have
>> cost_index untangle that case into a separate failure return so that
>> the within-the-core-optimizer APIs remain clean.
>>
>> While that would require hnsw to make a small code change (return
>> +Inf not DBL_MAX), that coding should work in back branches too,
>> so they don't even need a version check.
>
> +1 for this approach (I'll do a quick test in my pgvector workspace just
> to ensure it gets the same results in the older version).

...and I confirmed the +inf approach on PG16 +pgvector does still give
the same expected result.

Thanks,

Jonathan