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Robert Schulze 2023-01-22 16:45:26 +01:00
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docs/en/operations/settings/settings.md
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@ -410,49 +410,49 @@ Possible values:
- default - default
This is the equivalent of `hash` or `direct`, if possible (same as `direct,hash`) This is the equivalent of `hash` or `direct`, if possible (same as `direct,hash`)
- grace_hash - grace_hash
[Grace hash join](https://en.wikipedia.org/wiki/Hash_join#Grace_hash_join) is used. Grace hash provides an algorithm option that provides performant complex joins while limiting memory use. [Grace hash join](https://en.wikipedia.org/wiki/Hash_join#Grace_hash_join) is used. Grace hash provides an algorithm option that provides performant complex joins while limiting memory use.
The first phase of a grace join reads the right table and splits it into N buckets depending on the hash value of key columns (initially, N is `grace_hash_join_initial_buckets`). This is done in a way to ensure that each bucket can be processed independently. Rows from the first bucket are added to an in-memory hash table while the others are saved to disk. If the hash table grows beyond the memory limit (e.g., as set by [`max_bytes_in_join`](/docs/en/operations/settings/query-complexity.md/#settings-max_bytes_in_join)), the number of buckets is increased and the assigned bucket for each row. Any rows which don’t belong to the current bucket are flushed and reassigned. The first phase of a grace join reads the right table and splits it into N buckets depending on the hash value of key columns (initially, N is `grace_hash_join_initial_buckets`). This is done in a way to ensure that each bucket can be processed independently. Rows from the first bucket are added to an in-memory hash table while the others are saved to disk. If the hash table grows beyond the memory limit (e.g., as set by [`max_bytes_in_join`](/docs/en/operations/settings/query-complexity.md/#settings-max_bytes_in_join)), the number of buckets is increased and the assigned bucket for each row. Any rows which don’t belong to the current bucket are flushed and reassigned.
- hash - hash
[Hash join algorithm](https://en.wikipedia.org/wiki/Hash_join) is used. The most generic implementation that supports all combinations of kind and strictness and multiple join keys that are combined with `OR` in the `JOIN ON` section. [Hash join algorithm](https://en.wikipedia.org/wiki/Hash_join) is used. The most generic implementation that supports all combinations of kind and strictness and multiple join keys that are combined with `OR` in the `JOIN ON` section.
- parallel_hash - parallel_hash
A variation of `hash` join that splits the data into buckets and builds several hashtables instead of one concurrently to speed up this process. A variation of `hash` join that splits the data into buckets and builds several hashtables instead of one concurrently to speed up this process.
When using the `hash` algorithm, the right part of `JOIN` is uploaded into RAM. When using the `hash` algorithm, the right part of `JOIN` is uploaded into RAM.
- partial_merge - partial_merge
A variation of the [sort-merge algorithm](https://en.wikipedia.org/wiki/Sort-merge_join), where only the right table is fully sorted. A variation of the [sort-merge algorithm](https://en.wikipedia.org/wiki/Sort-merge_join), where only the right table is fully sorted.
The `RIGHT JOIN` and `FULL JOIN` are supported only with `ALL` strictness (`SEMI`, `ANTI`, `ANY`, and `ASOF` are not supported). The `RIGHT JOIN` and `FULL JOIN` are supported only with `ALL` strictness (`SEMI`, `ANTI`, `ANY`, and `ASOF` are not supported).
When using the `partial_merge` algorithm, ClickHouse sorts the data and dumps it to the disk. The `partial_merge` algorithm in ClickHouse differs slightly from the classic realization. First, ClickHouse sorts the right table by joining keys in blocks and creates a min-max index for sorted blocks. Then it sorts parts of the left table by the `join key` and joins them over the right table. The min-max index is also used to skip unneeded right table blocks. When using the `partial_merge` algorithm, ClickHouse sorts the data and dumps it to the disk. The `partial_merge` algorithm in ClickHouse differs slightly from the classic realization. First, ClickHouse sorts the right table by joining keys in blocks and creates a min-max index for sorted blocks. Then it sorts parts of the left table by the `join key` and joins them over the right table. The min-max index is also used to skip unneeded right table blocks.
- direct - direct
This algorithm can be applied when the storage for the right table supports key-value requests. This algorithm can be applied when the storage for the right table supports key-value requests.
The `direct` algorithm performs a lookup in the right table using rows from the left table as keys. It's supported only by special storage such as [Dictionary](../../engines/table-engines/special/dictionary.md/#dictionary) or [EmbeddedRocksDB](../../engines/table-engines/integrations/embedded-rocksdb.md) and only the `LEFT` and `INNER` JOINs. The `direct` algorithm performs a lookup in the right table using rows from the left table as keys. It's supported only by special storage such as [Dictionary](../../engines/table-engines/special/dictionary.md/#dictionary) or [EmbeddedRocksDB](../../engines/table-engines/integrations/embedded-rocksdb.md) and only the `LEFT` and `INNER` JOINs.
- auto - auto
When set to `auto`, `hash` join is tried first, and the algorithm is switched on the fly to another algorithm if the memory limit is violated. When set to `auto`, `hash` join is tried first, and the algorithm is switched on the fly to another algorithm if the memory limit is violated.
- full_sorting_merge - full_sorting_merge
[Sort-merge algorithm](https://en.wikipedia.org/wiki/Sort-merge_join) with full sorting joined tables before joining. [Sort-merge algorithm](https://en.wikipedia.org/wiki/Sort-merge_join) with full sorting joined tables before joining.
- prefer_partial_merge - prefer_partial_merge
ClickHouse always tries to use `partial_merge` join if possible, otherwise, it uses `hash`. *Deprecated*, same as `partial_merge,hash`. ClickHouse always tries to use `partial_merge` join if possible, otherwise, it uses `hash`. *Deprecated*, same as `partial_merge,hash`.
## join_any_take_last_row {#settings-join_any_take_last_row} ## join_any_take_last_row {#settings-join_any_take_last_row}