feat(dgraph): Add suport for RDF query.

[poonai]

This PR will allow users to get the query results in RDF format.
Implementation Details: https://discuss.dgraph.io/t/rfc-streaming-rdf-queries/8410

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This change is 

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[all-seeing-code]

We can remove these changes from the merge, right?

[poonai]

Yes

[MichelDiz]

Reviewable status: 0 of 6 files reviewed, 2 unresolved discussions (waiting on @ashish-goswami, @balajijinnah, @manishrjain, @parasssh, @pawanrawal, and @vvbalaji-dgraph)

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query/rdf_result_test.go, line 40 at r1 (raw file):

	require.NoError(t, err)
	require.Equal(t, string(rdf), `<0x1> <name> "Michonne" 
<0x1> <friend> <0x17> 

For some reason, I'm not seeing the dots at the end of each RDF line. Is it right?

[manishrjain]

Address my comments, please.

Reviewed 4 of 6 files at r1, 2 of 2 files at r2.
Reviewable status: all files reviewed, 13 unresolved discussions (waiting on @anurags92, @ashish-goswami, @balajijinnah, @parasssh, @pawanrawal, and @vvbalaji-dgraph)

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query/outputrdf.go, line 15 at r2 (raw file):

 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

Need a space here.

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query/outputrdf.go, line 29 at r2 (raw file):

// ToRDF converts the given subgraph list into rdf format.
func ToRDF(l *Latency, sgl []*SubGraph) ([]byte, error) {
	output := ""

Use a bytes.Buffer or something.

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query/outputrdf.go, line 37 at r2 (raw file):

			}
		}

vertical space.

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query/outputrdf.go, line 39 at r2 (raw file):

	}
	return []byte(output), nil

Avoid this allocation.

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query/outputrdf.go, line 44 at r2 (raw file):

// castToRDF converts the given subgraph to RDF and appends to the
// output string.
func castToRDF(output *string, sg *SubGraph) error {

I'd convert all these funcs into methods. And the type struct can hold the bytes.Buffer.

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query/outputrdf.go, line 45 at r2 (raw file):

// output string.
func castToRDF(output *string, sg *SubGraph) error {

Vertical space.

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query/outputrdf.go, line 50 at r2 (raw file):

		return errors.New("uid count is not supported in the rdf output format")
	}

here.

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query/outputrdf.go, line 72 at r2 (raw file):

	// Recursively cnvert RDF for the children graph.
	for _, child := range sg.Children {
		err := castToRDF(output, child)

if err := ; err != nil { ... }

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query/outputrdf.go, line 102 at r2 (raw file):

				return err
			}
			*output += fmt.Sprintf("<%#x> <%s> %s .\n", uid, sg.aggWithVarFieldName(),

fmt.Sprintf is very expensive.

Use bytes.Buffer.

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query/outputrdf.go, line 103 at r2 (raw file):

			}
			*output += fmt.Sprintf("<%#x> <%s> %s .\n", uid, sg.aggWithVarFieldName(),
				string(outputval))

buf.Write(outputval)

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query/outputrdf.go, line 128 at r2 (raw file):

		fieldName = fmt.Sprintf("count(%s)", sg.Attr)
	}
	*output += fmt.Sprintf("<%#x> <%s> %d .\n", subject, fieldName, count)

small helper methods, which you can give the subject, pred, object to, and it would just write it to buf.

[poonai]

Reviewable status: 1 of 6 files reviewed, 13 unresolved discussions (waiting on @anurags92, @ashish-goswami, @balajijinnah, @manishrjain, @MichelDiz, @parasssh, @pawanrawal, and @vvbalaji-dgraph)

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query/outputrdf.go, line 15 at r2 (raw file):

Previously, manishrjain (Manish R Jain) wrote…

Need a space here.

Done.

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query/outputrdf.go, line 29 at r2 (raw file):

Previously, manishrjain (Manish R Jain) wrote…

Use a bytes.Buffer or something.

Done.

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query/outputrdf.go, line 39 at r2 (raw file):

Previously, manishrjain (Manish R Jain) wrote…

Avoid this allocation.

Done.

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query/outputrdf.go, line 44 at r2 (raw file):

Previously, manishrjain (Manish R Jain) wrote…

I'd convert all these funcs into methods. And the type struct can hold the bytes.Buffer.

Done.

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query/outputrdf.go, line 45 at r2 (raw file):

Previously, manishrjain (Manish R Jain) wrote…

Vertical space.

Done.

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query/outputrdf.go, line 50 at r2 (raw file):

Previously, manishrjain (Manish R Jain) wrote…

here.

Done.

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query/outputrdf.go, line 72 at r2 (raw file):

Previously, manishrjain (Manish R Jain) wrote…

if err := ; err != nil { ... }

Done.

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query/outputrdf.go, line 102 at r2 (raw file):

Previously, manishrjain (Manish R Jain) wrote…

fmt.Sprintf is very expensive.

Use bytes.Buffer.

Done.

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query/outputrdf.go, line 103 at r2 (raw file):

Previously, manishrjain (Manish R Jain) wrote…

buf.Write(outputval)

Done.

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query/outputrdf.go, line 128 at r2 (raw file):

Previously, manishrjain (Manish R Jain) wrote…

small helper methods, which you can give the subject, pred, object to, and it would just write it to buf.

Done.

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query/rdf_result_test.go, line 40 at r1 (raw file):

Previously, MichelDiz (Michel Diz) wrote…

For some reason, I'm not seeing the dots at the end of each RDF line. Is it right?

Done.

[pawanrawal]

after the bug is fixed

Reviewable status: 1 of 6 files reviewed, 15 unresolved discussions (waiting on @anurags92, @ashish-goswami, @balajijinnah, @manishrjain, @MichelDiz, @parasssh, and @vvbalaji-dgraph)

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query/common_test.go, line 78 at r3 (raw file):

	res, err := txn.Do(ctx, &api.Request{
		Query:     query,
		RdfFormat: true,

We could call this as RespFormat and the value can be rdf. Default is json if nothing is given.

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query/rdf_result_test.go, line 46 at r3 (raw file):

<0x18> <name> "Glenn Rhee" .
<0x19> <name> "Daryl Dixon" .
<0x17> <age> 15 .

"15"
All the values have to be string literal. Just verify other types also like boolean, float and datetime.