Fix a bug in the cache key of DISTINCT

[joka921]

The cache key of a DISTINCT operation previously did not change when the subtree stayed the same, but the variables on which the DISTINCT was performed changed. For example, when first executing SELECT DISTINCT ?x ?y WHERE { ?x wdt:P30 ?y } and then SELECT DISTINCT ?x WHERE { ?x wdt:P30 ? }, the result for the second query would be mistakenly taken from the cache, although those queries have different results. This is now fixed by adding the column indices of the variables to the cache key.

[codecov]

Codecov Report

Attention: Patch coverage is 71.42857% with 4 lines in your changes missing coverage. Please review.

Project coverage is 89.14%. Comparing base (6a6a4b8) to head (abc3653).

Files	Patch %	Lines
src/engine/Distinct.h	62.50%	3 Missing ⚠️
src/engine/Distinct.cpp	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1403      +/-   ##
==========================================
- Coverage   89.14%   89.14%   -0.01%     
==========================================
  Files         332      332              
  Lines       29498    29497       -1     
  Branches     3291     3291              
==========================================
- Hits        26296    26295       -1     
  Misses       2050     2050              
  Partials     1152     1152              

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[hannahbast]

Thanks a lot for the fix!