ed73ed83d3
## What changes were proposed in this pull request? Document Explain statement in SQL Reference Guide. ## Why are the changes needed? Adding documentation for SQL reference. ## Does this PR introduce any user-facing change? yes Before: There was no documentation for this. After:    ## How was this patch tested? Used jekyll build and serve to verify. Closes #26970 from PavithraRamachandran/explain_doc. Authored-by: Pavithra Ramachandran <pavi.rams@gmail.com> Signed-off-by: Takeshi Yamamuro <yamamuro@apache.org>
140 lines
4.5 KiB
Markdown
140 lines
4.5 KiB
Markdown
---
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layout: global
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title: EXPLAIN
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displayTitle: EXPLAIN
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license: |
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Licensed to the Apache Software Foundation (ASF) under one or more
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contributor license agreements. See the NOTICE file distributed with
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this work for additional information regarding copyright ownership.
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The ASF licenses this file to You under the Apache License, Version 2.0
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(the "License"); you may not use this file except in compliance with
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the License. You may obtain a copy of the License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing, software
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distributed under the License is distributed on an "AS IS" BASIS,
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WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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See the License for the specific language governing permissions and
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limitations under the License.
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---
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### Description
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The `EXPLAIN` statement is used to provide logical/physical plans for an input statement.
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By default, this clause provides information about a physical plan only.
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### Syntax
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{% highlight sql %}
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EXPLAIN [EXTENDED | CODEGEN | COST | FORMATTED] statement
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{% endhighlight %}
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### Parameters
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<dl>
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<dt><code><em>EXTENDED</em></code></dt>
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<dd>Generates parsed logical plan, analyzed logical plan, optimized logical plan and physical plan.
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Parsed Logical plan is a unresolved plan that extracted from the query.
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Analyzed logical plans transforms which translates unresolvedAttribute and unresolvedRelation into fully typed objects.
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The optimized logical plan transforms through a set of optimization rules, resulting in the physical plan.
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</dd>
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</dl>
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<dl>
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<dt><code><em>CODEGEN</em></code></dt>
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<dd>Generates code for the statement, if any and a physical plan.</dd>
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</dl>
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<dl>
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<dt><code><em>COST</em></code></dt>
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<dd>If plan node statistics are available, generates a logical plan and the statistics.</dd>
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</dl>
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<dl>
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<dt><code><em>FORMATTED</em></code></dt>
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<dd>Generates two sections: a physical plan outline and node details.</dd>
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</dl>
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<dl>
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<dt><code><em>statement</em></code></dt>
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<dd>
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Specifies a SQL statement to be explained.
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</dd>
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</dl>
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### Examples
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{% highlight sql %}
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--Default Output
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EXPLAIN select k, sum(v) from values (1, 2), (1, 3) t(k, v) group by k;
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+----------------------------------------------------+
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| plan |
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+----------------------------------------------------+
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| == Physical Plan ==
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*(2) HashAggregate(keys=[k#33], functions=[sum(cast(v#34 as bigint))])
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+- Exchange hashpartitioning(k#33, 200), true, [id=#59]
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+- *(1) HashAggregate(keys=[k#33], functions=[partial_sum(cast(v#34 as bigint))])
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+- *(1) LocalTableScan [k#33, v#34]
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+----------------------------------------------------
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-- Using Extended
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EXPLAIN EXTENDED select k, sum(v) from values (1, 2), (1, 3) t(k, v) group by k;
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+----------------------------------------------------+
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| plan |
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+----------------------------------------------------+
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| == Parsed Logical Plan ==
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'Aggregate ['k], ['k, unresolvedalias('sum('v), None)]
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+- 'SubqueryAlias `t`
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+- 'UnresolvedInlineTable [k, v], [List(1, 2), List(1, 3)]
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== Analyzed Logical Plan ==
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k: int, sum(v): bigint
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Aggregate [k#47], [k#47, sum(cast(v#48 as bigint)) AS sum(v)#50L]
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+- SubqueryAlias `t`
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+- LocalRelation [k#47, v#48]
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== Optimized Logical Plan ==
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Aggregate [k#47], [k#47, sum(cast(v#48 as bigint)) AS sum(v)#50L]
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+- LocalRelation [k#47, v#48]
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== Physical Plan ==
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*(2) HashAggregate(keys=[k#47], functions=[sum(cast(v#48 as bigint))], output=[k#47, sum(v)#50L])
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+- Exchange hashpartitioning(k#47, 200), true, [id=#79]
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+- *(1) HashAggregate(keys=[k#47], functions=[partial_sum(cast(v#48 as bigint))], output=[k#47, sum#52L])
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+- *(1) LocalTableScan [k#47, v#48]
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+----------------------------------------------------+
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--Using Formatted
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EXPLAIN FORMATTED select k, sum(v) from values (1, 2), (1, 3) t(k, v) group by k;
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+----------------------------------------------------+
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| plan |
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+----------------------------------------------------+
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| == Physical Plan ==
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* HashAggregate (4)
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+- Exchange (3)
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+- * HashAggregate (2)
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+- * LocalTableScan (1)
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(1) LocalTableScan [codegen id : 1]
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Output: [k#19, v#20]
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(2) HashAggregate [codegen id : 1]
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Input: [k#19, v#20]
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(3) Exchange
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Input: [k#19, sum#24L]
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(4) HashAggregate [codegen id : 2]
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Input: [k#19, sum#24L]
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+----------------------------------------------------+
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{% endhighlight %}
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