[IMPALA-2581] Push down LIMIT past DISTINCT

• Type: Improvement
• Status: Resolved
• Priority: Minor
• Resolution: Done
• Affects Version/s: Impala 2.5.0
• Fix Version/s: Impala 4.1.0
• Component/s: Frontend
• Labels: backend frontend performance
• Target Version: Product Backlog
• Assignee: liuyao
• Reporter: Jim Apple
• Created: 22/Oct/15 15:54
• Updated: 15/Sep/23 18:38
• Resolved: 23/Sep/21 02:57

Description

In a table t with a column x with no null values, "SELECT DISTINCT x FROM t LIMIT 1" should be roughly instant. Instead, it finds all the distinct values, then returns one of them.

Issue Links

• causes: IMPALA-12383 Aggregation with num_nodes=1 and limit returns too many rows