[DRILL-6881] Hash-Table insert and probe: Compare hash values before keys

• Type: Improvement
• Status: Open
• Priority: Major
• Resolution: Unresolved
• Affects Version/s: 1.14.0
• Component/s: Execution - Relational Operators
• Assignee: Boaz Ben-Zvi
• Reporter: Boaz Ben-Zvi
• Created: 05/Dec/18 05:17
• Updated: 12/Dec/18 22:27

Description

  When checking for existence of a key in the hash table (during put or probe operations), the value of that key is compared (using generated code) with a potential match key (same bucket).
   This comparison is slightly expensive (e.g., long keys, multi column keys, checking null conditions, NaN, etc). Instead, if the hash-values of the two keys are compared first (at practically zero cost), then the costly comparison can be avoided in case the hash values don't match.
This code change is trivial, and given that the relevant Hash-Table code is hot code, then even minute improvements could add up.