179289f0bf
### What changes were proposed in this pull request?
This PR intends to clean up the SQL documents in `doc/sql-ref*`.
Main changes are as follows;
- Fixes wrong syntaxes and capitalize sub-titles
- Adds some DDL queries in `Examples` so that users can run examples there
- Makes query output in `Examples` follows the `Dataset.showString` (right-aligned) format
- Adds/Removes spaces, Indents, or blank lines to follow the format below;
```
---
license...
---
### Description
Writes what's the syntax is.
### Syntax
{% highlight sql %}
SELECT...
WHERE... // 4 indents after the second line
...
{% endhighlight %}
### Parameters
<dl>
<dt><code><em>Param Name</em></code></dt>
<dd>
Param Description
</dd>
...
</dl>
### Examples
{% highlight sql %}
-- It is better that users are able to execute example queries here.
-- So, we prepare test data in the first section if possible.
CREATE TABLE t (key STRING, value DOUBLE);
INSERT INTO t VALUES
('a', 1.0), ('a', 2.0), ('b', 3.0), ('c', 4.0);
-- query output has 2 indents and it follows the `Dataset.showString`
-- format (right-aligned).
SELECT * FROM t;
+---+-----+
|key|value|
+---+-----+
| a| 1.0|
| a| 2.0|
| b| 3.0|
| c| 4.0|
+---+-----+
-- Query statements after the second line have 4 indents.
SELECT key, SUM(value)
FROM t
GROUP BY key;
+---+----------+
|key|sum(value)|
+---+----------+
| c| 4.0|
| b| 3.0|
| a| 3.0|
+---+----------+
...
{% endhighlight %}
### Related Statements
* [XXX](xxx.html)
* ...
```
### Why are the changes needed?
The most changes of this PR are pretty minor, but I think the consistent formats/rules to write documents are important for long-term maintenance in our community
### Does this PR introduce any user-facing change?
Yes.
### How was this patch tested?
Manually checked.
Closes #28151 from maropu/MakeRightAligned.
Authored-by: Takeshi Yamamuro <yamamuro@apache.org>
Signed-off-by: Sean Owen <srowen@gmail.com>
4 KiB
| layout | title | displayTitle | license |
|---|---|---|---|
| global | Join Hints | Join Hints | Licensed to the Apache Software Foundation (ASF) under one or more contributor license agreements. See the NOTICE file distributed with this work for additional information regarding copyright ownership. The ASF licenses this file to You under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at http://www.apache.org/licenses/LICENSE-2.0 Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License. |
Description
Join Hints allow users to suggest the join strategy that Spark should use. Prior to Spark 3.0, only the BROADCAST Join Hint was supported. MERGE, SHUFFLE_HASH and SHUFFLE_REPLICATE_NL Joint Hints support was added in 3.0. When different join strategy hints are specified on both sides of a join, Spark prioritizes hints in the following order: BROADCAST over MERGE over SHUFFLE_HASH over SHUFFLE_REPLICATE_NL. When both sides are specified with the BROADCAST hint or the SHUFFLE_HASH hint, Spark will pick the build side based on the join type and the sizes of the relations. Since a given strategy may not support all join types, Spark is not guaranteed to use the join strategy suggested by the hint.
Join Hints Types
BROADCAST-
Suggests that Spark use broadcast join. The join side with the hint will be broadcast regardless of
autoBroadcastJoinThreshold. If both sides of the join have the broadcast hints, the one with the smaller size (based on stats) will be broadcast. The aliases forBROADCASTareBROADCASTJOINandMAPJOIN.
MERGE-
Suggests that Spark use shuffle sort merge join. The aliases for
MERGEareSHUFFLE_MERGEandMERGEJOIN.
SHUFFLE_HASH- Suggests that Spark use shuffle hash join. If both sides have the shuffle hash hints, Spark chooses the smaller side (based on stats) as the build side.
SHUFFLE_REPLICATE_NL- Suggests that Spark use shuffle-and-replicate nested loop join.
Examples
{% highlight sql %} -- Join Hints for broadcast join SELECT /*+ BROADCAST(t1) / * FROM t1 INNER JOIN t2 ON t1.key = t2.key; SELECT /+ BROADCASTJOIN (t1) / * FROM t1 left JOIN t2 ON t1.key = t2.key; SELECT /+ MAPJOIN(t2) */ * FROM t1 right JOIN t2 ON t1.key = t2.key;
-- Join Hints for shuffle sort merge join SELECT /*+ SHUFFLE_MERGE(t1) / * FROM t1 INNER JOIN t2 ON t1.key = t2.key; SELECT /+ MERGEJOIN(t2) / * FROM t1 INNER JOIN t2 ON t1.key = t2.key; SELECT /+ MERGE(t1) */ * FROM t1 INNER JOIN t2 ON t1.key = t2.key;
-- Join Hints for shuffle hash join SELECT /*+ SHUFFLE_HASH(t1) */ * FROM t1 INNER JOIN t2 ON t1.key = t2.key;
-- Join Hints for shuffle-and-replicate nested loop join SELECT /*+ SHUFFLE_REPLICATE_NL(t1) */ * FROM t1 INNER JOIN t2 ON t1.key = t2.key;
-- When different join strategy hints are specified on both sides of a join, Spark -- prioritizes the BROADCAST hint over the MERGE hint over the SHUFFLE_HASH hint -- over the SHUFFLE_REPLICATE_NL hint. -- Spark will issue Warning in the following example -- org.apache.spark.sql.catalyst.analysis.HintErrorLogger: Hint (strategy=merge) -- is overridden by another hint and will not take effect. SELECT /*+ BROADCAST(t1) / /+ MERGE(t1, t2) */ * FROM t1 INNER JOIN t2 ON t1.key = t2.key; {% endhighlight %}