[SQL][Feature Request] Infer Output schema

*From: https://stackoverflow.com/questions/52181795/how-do-i-get-an-output-schema-for-an-apache-beam-sql-query :*

I've been playing with the Beam SQL DSL and I'm unable to use the output from a query without providing a code that's aware of the output schema manually. Can I infer the output schema rather than hardcoding it?

Neither the walkthrough or the examples actually use the output from a query. I'm using Scio rather than the plain Java API to keep the code relatively readable and concise, I don't think that makes a difference for this question.

Here's an example of what I mean.

Given an input schema inSchema and some data source that is mapped onto a Row as follows: (in this example, Avro-based, but again, I don't think that matters):

```

sc.avroFile[Foo](args("input"))
.map(fooToRow)
.setCoder(inSchema.getRowCoder)
.applyTransform(SqlTransform.query("SELECT
COUNT(1) FROM PCOLLECTION"))
.saveAsTextFile(args("output"))

```

Running this pipeline results in a KryoException as follows:

```

com.esotericsoftware.kryo.KryoException: java.lang.NullPointerException
Serialization trace:
fieldIndices
(org.apache.beam.sdk.schemas.Schema)
schema (org.apache.beam.sdk.values.RowWithStorage)
org.apache.beam.sdk.Pipeline$PipelineExecutionException:

com.esotericsoftware.kryo.KryoException: java.lang.NullPointerException

```

However, inserting a RowCoder matching the SQL output, in this case a single count int column:

```

...snip...
.applyTransform(SqlTransform.query("SELECT COUNT(1) FROM PCOLLECTION"))
.setCoder(Schema.builder().addInt64Field("count").build().getRowCoder)

.saveAsTextFile(args("output"))

```

Now the pipeline runs just fine.

Having to manually tell the pipeline how to encode the SQL output seems unnecessary, given that we specify the input schema/coder(s) and a query. It seems to me that we should be able to infer the output schema from that - but I can't see how, other than maybe using Calcite directly?

Before raising a ticket on the Beam Jira, I thought I'd check I wasn't missing something obvious!

Imported from Jira [BEAM-5335](https://issues.apache.org/jira/browse/BEAM-5335). Original Jira may contain additional context.
Reported by: kedin.