@antepusic move description to comment section

Description

Context

If the following two conditions are met, the result of an aggregation is an empty set (cf. #1531):

1. there’s no input to aggregate over
2. results are returned grouped

Issue #1582 (the query MATCH (n:DoesNotExist) RETURN count(*) > 0 AS exists; returning an empty set) was caused by Memgraph mistakenly identifying the 0 constant as a grouping key.

Changes

This PR fixes the issue by excluding constants from the grouping keys, in line with Cypher:

For better query readability, Cypher only recognizes a sub-expression in aggregating expressions as a grouping key if the grouping key is either:

• A variable - e.g. the p in RETURN p, p.age - max(f.age).
• A property access - e.g. the p.age in RETURN p.age, p.age - max(f.age).
• A map access - e.g. the p.age in WITH {name:'Keanu Reeves', age:58} AS p RETURN p.age, p.age - max(p.age).
  (from Neo4j docs)

[master < Task] PR

• Provide the full content or a guide for the final git message

  Exclude constants from grouping keys

Documentation checklist

• Add the documentation label tag
• Add the bug / feature label tag
• Add the milestone for which this feature is intended
• Write a release note, including added/changed clauses

  Constant values aren’t used anymore to group the values going into aggregation functions. Exclude constants from grouping keys #2095

• Link the documentation PR here (N/A – changelog only)
• Tag someone from docs team in the comments

@kgolubic Find the changelog entry in #2095 (comment)

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

For release note I'll use:

• Resolved an issue where constants were incorrectly used as grouping keys in aggregation queries.
  #2095