Parallel Query Execution in PostgreSQL

PostgreSQL employs a Parallel Query feature that optimizes the execution of certain queries, boosting performance in multi-core systems.

Understanding Parallel Query

• Parallel Query is invoked when the optimizer deems it the most efficient strategy.
• It employs multiple background worker processes for concurrent execution.

The Plan Structure

• A query plan with Parallel Query includes a Gather or Gather Merge node.
• The Gather node assembles the parallel execution results.

Parallel Execution Scope

• If the Gather node is at the plan's top, the entire query runs in parallel.
• If within the plan tree, only the nodes below it run in parallel.

Optimal Worker Count

• The planner determines the number of workers.
• The chosen workers execute the parallel portion of the query.
• The number of workers can be capped by max_parallel_workers_per_gather.

Leader's Role

• The session's main process becomes the leader.
• The leader reads the tuples generated by the workers.
• For small result sets, the leader assists in query speed-up.

Gather Merge Insight

• When using Gather Merge nodes, workers produce sorted tuples.
• The leader merges and maintains the sort order.

Conclusion

PostgreSQL's Parallel Query mechanism is a powerful tool for enhancing query performance. By leveraging multiple background workers, parallel execution optimizes data retrieval, particularly on multi-core systems. Through careful planning and an understanding of parallel execution's nuances, developers can harness this feature to accelerate query processing, delivering improved responsiveness and efficiency.

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parallel query