Re: Parameter for planner estimate of recursive queries

On Wed, 27 Oct 2021 at 15:58, Simon Riggs <simon.riggs@enterprisedb.com> wrote:

> The poor performance is traced to the planner cost estimates for
> recursive queries. Specifically, the cost of the recursive arm of the
> query is evaluated based upon both of these hardcoded assumptions:
>
> 1. The recursion will last for 10 loops
> 2. The average size of the worktable will be 10x the size of the
> initial query (non-recursive term).
>
> Taken together these assumptions lead to a very poor estimate of the
> worktable activity (in this case), which leads to the plan changing as
> a result.
>
> 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.

So I'm, thinking we use

rel->tuples = min(1, cte_rows * cte_rows/2);

-- 
Simon Riggs                http://www.EnterpriseDB.com/