mirror of https://github.com/citusdata/citus.git
c1f5762645
DESCRIPTION: Ensure that a MERGE command on a distributed table with a `WHEN NOT MATCHED BY SOURCE` clause runs against all shards of the distributed table. The Postgres MERGE command updates a table using a table or a query as a data source. It provides three ways to match the target table with the source: `WHEN MATCHED` means that there is a row in both the target and source; `WHEN NOT MATCHED` means that there is a row in the source that has no match (is not present) in the target; and, as of PG17, `WHEN NOT MATCHED BY SOURCE` means that there is a row in the target that has no match in the source. In Citus, when a MERGE command updates a distributed table using a local/reference table or a distributed query as source, that source is repartitioned, and for each repartitioned shard that has data (i.e. 1 or more rows) the MERGE is run against the corresponding distributed table shard. Suppose the distributed table has 32 shards, and the source repartitions into 4 shards that have data, with the remaining 28 shards being empty; then the MERGE command is performed on the 4 corresponding shards of the distributed table. However, the semantics of `WHEN NOT MATCHED BY SOURCE` are that the specified action must be performed on the target for each row in the target that is not in the source; so if the source is empty, all target rows should be updated. To see this, consider the following MERGE command: ``` MERGE INTO target AS t USING source AS s ON t.id = s.id WHEN NOT MATCHED BY SOURCE THEN UPDATE t SET t.col1 = 100 ``` If the source has zero rows then every row in the target is updated s.t. its col1 value is 100. Currently in Citus a MERGE on a distributed table with a local/reference table or a distributed query as source ignores shards of the distributed table when the corresponding shard of the repartitioned source has zero rows. However, if the MERGE command specifies a `WHEN NOT MATCHED BY SOURCE` clause, then the MERGE should be performed on all shards of the distributed table, to ensure that the specified action is performed on the target for each row in the target that is not in the source. This PR enhances Citus MERGE execution so that when a repartitioned source shard has zero rows, and the MERGE command specifies a `WHEN NOT MATCHED BY SOURCE` clause, the MERGE is performed against the corresponding shard of the distributed table using an empty (zero row) relation as source, by generating a query of the form: ``` MERGE INTO target_shard_0002 AS t USING (SELECT id FROM (VALUES (NULL) ) source_0002(id) WHERE FALSE) AS s ON t.id = s.id WHEN NOT MATCHED BY SOURCE THEN UPDATE t set t.col1 = 100 ``` This works because each row in the target shard will be updated, and `WHEN MATCHED` and `WHEN NOT MATCHED`, if specified, will be no-ops because the source has zero rows. To implement this when the source is a local or reference table involves teaching function `ExcuteSourceAtCoordAndRedistribution()` in `merge_executor.c` to not prune tasks when the query has `WHEN NOT MATCHED BY SOURCE` but to instead replace the task's query to one that uses an empty relation as source. And when the source is a distributed query, function `ExecuteMergeSourcePlanIntoColocatedIntermediateResults()` (also in `merge_executor.c`) instead of skipping empty tasks now generates a query that uses an empty relation as source for the corresponding target shard of the distributed table, but again only when the query has `WHEN NOT MATCHED BY SOURCE`. A new function `BuildEmptyResultQuery()` is added to `recursive_planning.c` and it is used by both the aforementioned functions in `merge_executor.c` to build an empty relation to use as the source. It applies the appropriate type to each column of the empty relation so the join with the target makes sense to the query compiler. |
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| .. | ||
| adaptive_executor.c | ||
| citus_custom_scan.c | ||
| directed_acyclic_graph_execution.c | ||
| distributed_execution_locks.c | ||
| distributed_intermediate_results.c | ||
| executor_util_params.c | ||
| executor_util_tasks.c | ||
| executor_util_tuples.c | ||
| insert_select_executor.c | ||
| intermediate_results.c | ||
| local_executor.c | ||
| merge_executor.c | ||
| multi_executor.c | ||
| multi_server_executor.c | ||
| partitioned_intermediate_results.c | ||
| placement_access.c | ||
| query_stats.c | ||
| repartition_executor.c | ||
| repartition_join_execution.c | ||
| subplan_execution.c | ||
| transmit.c | ||
| tuple_destination.c | ||