[SPARK-30658] Limit after on streaming dataframe before streaming agg returns wrong results

• Type: Bug
• Status: Resolved
• Priority: Critical
• Resolution: Fixed
• Affects Version/s: 2.3.0, 2.3.1, 2.3.2, 2.3.3, 2.3.4, 2.4.0, 2.4.1, 2.4.2, 2.4.3, 2.4.4
• Fix Version/s: 3.0.0
• Component/s: Structured Streaming
• Target Version/s: 3.0.0
• Assignee: Tathagata Das
• Reporter: Tathagata Das
• Created: 28/Jan/20 09:12
• Updated: 01/Feb/20 00:42
• Resolved: 01/Feb/20 00:29

Description

Limit before a streaming aggregate (i.e. df.limit(5).groupBy().count()) in complete mode was not being planned as a streaming limit. The planner rule planned a logical limit with a stateful streaming limit plan only if the query is in append mode. As a result, instead of allowing max 5 rows across batches, the planned streaming query was allowing 5 rows in every batch thus producing incorrect results.