DESCRIPTION: Avoid query deparse and planning of shard query in local execution. Adds citus.enable_local_execution_local_plan GUC to allow avoiding unnecessary query deparsing to improve performance of fast-path queries targeting local shards.
If a fast path query resolves to a shard that is local to the node planning the query, a shortcut can be taken so that the OID of the shard is plugged into the parse tree, which is then planned by Postgres. In `local_executor.c` the task uses that plan instead of parsing and planning a shard query. How this is done: The fast path planner identifies if the shortcut is possible, and then the distributed planner checks, using `CheckAndBuildDelayedFastPathPlan()`, if a local plan can be generated or if the shard query should be generated.
This optimization is controlled by a GUC `citus.enable_local_execution_local_plan` which is on by default. A new
regress test `local_execution_local_plan` tests both row-sharding and schema sharding. Negative tests are added to
`local_shard_execution_dropped_column` to verify that the optimization is not taken when the shard is local but there is a difference between the shard and distributed table because of a dropped column.
Colm