Re: Parameter for planner estimate of recursive queries

On 31.12.21 15:10, Simon Riggs wrote:
>> The factor 10 is a reasonably safe assumption and helps avoid worst
>> case behavior in bigger graph queries. However, the factor 10 is way
>> too large for many types of graph query, such as where the path
>> through the data is tight, and/or the query is written to prune bushy
>> graphs, e.g. shortest path queries. The factor 10 should not be
>> hardcoded in the planner, but should be settable, just as
>> cursor_tuple_fraction is.
> If you think this should be derived without parameters, then we would
> want a function that starts at 1 for 1 input row and gets much larger
> for larger input. The thinking here is that Graph OLTP is often a
> shortest path between two nodes, whereas Graph Analytics and so the
> worktable will get much bigger.

On the one hand, this smells like a planner hint. But on the other
hand, it doesn't look like we will come up with proper graph-aware
selectivity estimation system any time soon, so just having all graph
OLTP queries suck until then because the planner hint is hardcoded
doesn't seem like a better solution. So I think this setting can be ok.
I think the way you have characterized it makes sense, too: for graph
OLAP, you want a larger value, for graph OLTP, you want a smaller value.