[IMPALA-7969] Always admit trivial queries immediately

• Type: Improvement
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Fix Version/s: Impala 4.3.0
• Component/s: Backend
• Labels: admission-control
• Target Version: Impala 4.3.0
• Epic Color: ghx-label-1
• Assignee: Yida Wu
• Reporter: Tim Armstrong
• Created: 13/Dec/18 18:11
• Updated: 29/Jan/23 11:42
• Resolved: 29/Jan/23 11:42

Description

Here are two common query types that consume minimal resources:

• select ... from ... limit 0, which is used by some clients to determine column types
• select <expr1>, <expr2>, <expr3>, which just evaluates some constant expressions on the coordinator

Currently these queries get queued if there are existing queued queries or the number of queries limit is exceeded, which is inconvenient for use cases where latency is important. I think the planner should identify trivial queries and admission controller should admit immediately.

Here's an initial thought on the definition of a trivial query:

• Must have PLAN ROOT SINK as the root
• Can contain UNION and EMPTYSET nodes only