BIND clause slows down some queries

### Version

5.3.0

### Question

Dear fuseki community,

I have observed several times with different queries, that the presence of a simple BIND clause can drastically slows down its execution.

For example, the following query

```{sparql}
PREFIX rdfs:
PREFIX reconx:
PREFIX mnx:

SELECT
?mnet
?chem_1
?chem_1_label
?chem_2
?chem_2_label
WHERE{
{
SELECT DISTINCT ?mnet ?mnxm ?chem_1 ?chem_2
WHERE{
{
SELECT DISTINCT ?mnet ?mnxm ?chem_1 ?chem_2
WHERE{
BIND( reconx:vmh_Recon AS ?mnet ) # fix a model here to focus the results
?mnet reconx:reac/reconx:equaSource/reconx:part/reconx:spec/reconx:chem ?chem_1 .
?mnxm mnx:chemXref ?chem_1, ?chem_2
FILTER( STR( ?chem_1 ) < STR( ?chem_2 ))
}
}
?mnet reconx:reac/reconx:equaSource/reconx:part/reconx:spec/reconx:chem ?chem_2
}
}
?chem_1 reconx:label ?chem_1_label .
?chem_2 reconx:label ?chem_2_label .
}
```

takes at least half an hour to execute on my local fuseki instance.

But, if I remove the BIND clause from the most inner WHERE clause, by inlining the subject:

```{sparql}
...
WHERE{
# BIND( reconx:vmh_Recon AS ?mnet ) # fix a model here to focus the results
reconx:vmh_Recon reconx:reac/reconx:equaSource/reconx:part/reconx:spec/reconx:chem ?chem_1 .
?mnxm mnx:chemXref ?chem_1, ?chem_2
FILTER( STR( ?chem_1 ) < STR( ?chem_2 ))
}
...
```
the query now executes in a couple of second!

Replacing the BIND clause with a VALUES clause also executes very slowly.

The same query executed on GraphDB populated with the same dataset takes a couple of second to execute, with no significant differences between the three variants (inline, BIND, VALUES)

I tend to prefer to use the syntax with the explicit BIND or VALUES clause, because in a complex query it permits to syntactically highlight the "input parameter". But currently, the price is too high. I wonder it has to do with the query optimiser.

This being reported, thanks a lot for maintaining fuseki which is a great *open-source* tool.

Marco

Start by reproducing the inline, BIND, and VALUES query variants from the issue against Fuseki 5.3.0 and compare their execution behavior. No source file or test is named; trace the query-optimization path involved in constant bindings, and consider the work complete when equivalent variants perform comparably with a regression test.

Written by the indexing model from the issue text.