Github user cloud-fan commented on a diff in the pull request:
https://github.com/apache/spark/pull/19714#discussion_r153677668
--- Diff: docs/sql-programming-guide.md ---
@@ -1492,6 +1492,61 @@ that these options will be deprecated in future
release as more optimizations ar
</tr>
</table>
+## Broadcast Hint for SQL Queries
+
+Broadcast hint is a way for users to manually annotate a query and suggest
to the query optimizer the join method.
+It is very useful when the query optimizer cannot make optimal decision
with respect to join methods
+due to conservativeness or the lack of proper statistics.
+Spark Broadcast Hint has higher priority than autoBroadcastJoin mechanism,
examples:
+
+<div class="codetabs">
+
+<div data-lang="scala" markdown="1">
+
+{% highlight scala %}
+val src = sql("SELECT * FROM src")
+broadcast(src).join(recordsDF, Seq("key")).show()
+{% endhighlight %}
+
+</div>
+
+<div data-lang="java" markdown="1">
+
+{% highlight java %}
+Dataset<Row> src = sql("SELECT * FROM src");
+broadcast(src).join(recordsDF, Seq("key")).show();
+{% endhighlight %}
+
+</div>
+
+<div data-lang="python" markdown="1">
+
+{% highlight python %}
+src = spark.sql("SELECT * FROM src")
+recordsDF.join(broadcast(src), "key").show()
+{% endhighlight %}
+
+</div>
+
+<div data-lang="r" markdown="1">
+
+{% highlight r %}
+src <- sql("SELECT COUNT(*) FROM src")
+showDF(join(broadcast(src), recordsDF, src$key == recordsDF$key)))
+{% endhighlight %}
+
+</div>
+
+<div data-lang="sql" markdown="1">
+
+{% highlight sql %}
+SELECT /*+ BROADCAST(r) */ * FROM records r JOIN src s ON r.key = s.key
+{% endhighlight %}
+
+</div>
+</div>
+(Note that we accept `BROADCAST`, `BROADCASTJOIN` and `MAPJOIN` for
broadcast hint)
--- End diff --
shall we treat it as a comment on the SQL example?
```
--we accept BROADCAST, BROADCASTJOIN and MAPJOIN for broadcast hint
SELECT /*+ BROADCAST(r) */ * FROM records r JOIN src s ON r.key = s.key
```
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