Before this patch it wasn't possible to optimize simple SELECT * FROM
dist ORDER BY (w/o GROUP BY and DISTINCT) to more optimal stage
(QueryProcessingStage::WithMergeableStateAfterAggregationAndLimit),
since that code was under
allow_nondeterministic_optimize_skip_unused_shards, rework it and make
it possible.
Also now distributed_push_down_limit is respected for
optimize_distributed_group_by_sharding_key.
Next step will be to enable distributed_push_down_limit by default.
v2: fix detection of aggregates
- create directory monitors in parallel (this also includes rmdir in
case of directory is empty, since even if the directory is empty it
may take some time to remove it, due to waiting for journal or if the
directory is large, i.e. it had lots of files before, since remember
ext4 does not truncate the directory size on each unlink [1])
- initialize increment in parallel too (since it does readdir())
[1]: https://lore.kernel.org/linux-ext4/930A5754-5CE6-4567-8CF0-62447C97825C@dilger.ca/
Add two new settings for the Distributed engine:
- bytes_to_delay_insert
- max_delay_to_insert
If at the beginning of INSERT there will be too much pending data, more
then bytes_to_delay_insert, then the INSERT will wait until it will be
shrinked, and not more then max_delay_to_insert seconds.
If after this there will be still too much pending, it will throw an
exception.
Also new profile events were added (by analogy to the MergeTree):
- DistributedDelayedInserts (although you can use system.errors instead
of this, but still)
- DistributedRejectedInserts
- DistributedDelayedInsertsMilliseconds
Right now with distributed_directory_monitor_batch_inserts=1 and
insert_distributed_sync=0 INSERT into Distributed table will store
blocks that should be sent to remote (and in case of
prefer_localhost_replica=0 to the localhost too) on the local
filesystem, and sent it in background.
However there is no limit for this storage, and if the remote is
unavailable (or some other error), these pending blocks may take
significant space, and this is not always desired behaviour.
Add new Distributed setting - bytes_to_throw_insert, that will set the
limit for how much pending bytes is allowed, if the limit will be
reached an exception will be throw.
By default was set to 0, to avoid surprises.
Right now SYSTEM FLUSH DISTRIBUTED will block:
- INSERT into this Distributed table (requireDirectoryMonitor())
- SELECT * FROM system.distribution_queue
Two new settings (by analogy with MergeTree family) has been added:
- `fsync_after_insert` - Do fsync for every inserted. Will decreases
performance of inserts.
- `fsync_tmp_directory` - Do fsync for temporary directory (that is used
for async INSERT only) after all part operations (writes, renames,
etc.).
Refs: #17380 (p1)
* Distributed insertion to one random shard
* add some tests
* add some documentation
* Respect shards' weights
* fine locking
Co-authored-by: Ivan Lezhankin <ilezhankin@yandex-team.ru>
Previous set of QueryProcessingStage does not allow to do this.
But after WithMergeableStateAfterAggregation had been introduced the
following queries can be optimized too under
optimize_distributed_group_by_sharding_key:
- GROUP BY sharding_key LIMIT
- GROUP BY sharding_key LIMIT BY
- GROUP BY sharding_key ORDER BY
And right now it is still not supports:
- WITH TOTALS (looks like it can be supported)
- WITH ROLLUP (looks like it can be supported)
- WITH CUBE
- SETTINGS extremes=1 (looks like it can be supported)
But will be implemented separatelly.
vX: fixes
v2: fix WITH *
v3: fix extremes
v4: fix LIMIT OFFSET (and make a little bit cleaner)
v5: fix HAVING
v6: fix ORDER BY
v7: rebase against 20.7
v8: move out WithMergeableStateAfterAggregation
v9: add optimize_distributed_group_by_sharding_key into test names
