SQL: Add option to provide the delimiter for the CSV format

docs/reference/sql/endpoints/rest.asciidoc

@@ -15,7 +15,7 @@
 === Overview
 
 The SQL REST API accepts SQL in a JSON document, executes it,
-and returns the results. 
+and returns the results.
 For example:
 
 [source,console]
@@ -106,6 +106,21 @@ s|Description
 
 |===
 
+The `CSV` format accepts the following additional formatting URL query attributes:
+
+* `header`
++
+Controls if the CSV header should be included in the response or not. It can take one of the following values:
++
+   - `present` (default): include the header;
+   - `absent`: exclude the header.
++
+* `delimiter` (default: `,`)
++
+Indicates which character should be used to separate the CSV values. The following values cannot be used for this attribute:
+double quote (`"`), carriage-return (`\r`) and new-line (`\n`). The tab (`\t`) can also not be used, the `tsv` format needs to be used
+instead.
+
 Here are some examples for the human readable formats:
 
 ==== CSV
@@ -120,7 +135,7 @@ POST /_sql?format=csv
 --------------------------------------------------
 // TEST[setup:library]
 
-Which returns:
+which returns:
 
 [source,text]
 --------------------------------------------------
@@ -133,6 +148,30 @@ James S.A. Corey,Leviathan Wakes,561,2011-06-02T00:00:00.000Z
 --------------------------------------------------
 // TESTRESPONSE[non_json]
 
+or:
+
+[source,console]
+--------------------------------------------------
+POST /_sql?format=csv&header=absent&delimiter=%3b
+{
+    "query": "SELECT * FROM library ORDER BY page_count DESC",
+    "fetch_size": 5
+}
+--------------------------------------------------
+// TEST[setup:library]
+
+which returns:
+
+[source,text]
+--------------------------------------------------
+Peter F. Hamilton;Pandora's Star;768;2004-03-02T00:00:00.000Z
+Vernor Vinge;A Fire Upon the Deep;613;1992-06-01T00:00:00.000Z
+Frank Herbert;Dune;604;1965-06-01T00:00:00.000Z
+Alastair Reynolds;Revelation Space;585;2000-03-15T00:00:00.000Z
+James S.A. Corey;Leviathan Wakes;561;2011-06-02T00:00:00.000Z
+--------------------------------------------------
+// TESTRESPONSE[non_json]
+
 ==== JSON
 
 [source,console]
@@ -210,7 +249,7 @@ Which returns:
 
 [source,text]
 --------------------------------------------------
-     author      |        name        |  page_count   |      release_date      
+     author      |        name        |  page_count   |      release_date
 -----------------+--------------------+---------------+------------------------
 Peter F. Hamilton|Pandora's Star      |768            |2004-03-02T00:00:00.000Z
 Vernor Vinge     |A Fire Upon the Deep|613            |1992-06-01T00:00:00.000Z

x-pack/plugin/sql/sql-proto/src/main/java/org/elasticsearch/xpack/sql/proto/Protocol.java

@@ -50,15 +50,22 @@ public final class Protocol {
     public static final TimeValue PAGE_TIMEOUT = TimeValue.timeValueSeconds(45);
     public static final boolean FIELD_MULTI_VALUE_LENIENCY = false;
     public static final boolean INDEX_INCLUDE_FROZEN = false;
-    
+
     /*
-     * Using the Boolean object here so that SqlTranslateRequest to set this to null (since it doesn't need a "columnar" or 
+     * Using the Boolean object here so that SqlTranslateRequest to set this to null (since it doesn't need a "columnar" or
      * binary parameter).
      * See {@code SqlTranslateRequest.toXContent}
      */
     public static final Boolean COLUMNAR = Boolean.FALSE;
     public static final Boolean BINARY_COMMUNICATION = null;
 
+    /*
+     * URL parameters
+     */
+    public static final String URL_PARAM_FORMAT = "format";
+    public static final String URL_PARAM_HEADER = "header";
+    public static final String URL_PARAM_DELIMITER = "delimiter";
+
     /**
      * SQL-related endpoints
      */

x-pack/plugin/sql/src/main/java/org/elasticsearch/xpack/sql/plugin/RestSqlQueryAction.java

@@ -24,13 +24,20 @@
 
 import java.io.IOException;
 import java.nio.charset.StandardCharsets;
+import java.util.Collections;
 import java.util.List;
+import java.util.Set;
 
 import static org.elasticsearch.rest.RestRequest.Method.GET;
 import static org.elasticsearch.rest.RestRequest.Method.POST;
+import static org.elasticsearch.xpack.sql.proto.Protocol.URL_PARAM_DELIMITER;
+import static org.elasticsearch.xpack.sql.proto.Protocol.URL_PARAM_FORMAT;
+import static org.elasticsearch.xpack.sql.proto.Protocol.URL_PARAM_HEADER;
 
 public class RestSqlQueryAction extends BaseRestHandler {
 
+    TextFormat textFormat;
+
     @Override
     public List<Route> routes() {
         return List.of(
@@ -65,7 +72,7 @@ protected RestChannelConsumer prepareRequest(RestRequest request, NodeClient cli
             // enforce CBOR response for drivers and CLI (unless instructed differently through the config param)
             accept = XContentType.CBOR.name();
         } else {
-            accept = request.param("format");
+            accept = request.param(URL_PARAM_FORMAT);
         }
         if (accept == null) {
             accept = request.header("Accept");
@@ -87,7 +94,7 @@ protected RestChannelConsumer prepareRequest(RestRequest request, NodeClient cli
          * which we turn into a 400 error.
          */
         XContentType xContentType = accept == null ? XContentType.JSON : XContentType.fromMediaTypeOrFormat(accept);
-        TextFormat textFormat = xContentType == null ? TextFormat.fromMediaTypeOrFormat(accept) : null;
+        textFormat = xContentType == null ? TextFormat.fromMediaTypeOrFormat(accept) : null;
 
         if (xContentType == null && sqlRequest.columnar()) {
             throw new IllegalArgumentException("Invalid use of [columnar] argument: cannot be used in combination with "
@@ -124,6 +131,11 @@ public RestResponse buildResponse(SqlQueryResponse response) throws Exception {
         });
     }
 
+    @Override
+    protected Set<String> responseParams() {
+        return textFormat == TextFormat.CSV ? Set.of(URL_PARAM_DELIMITER, URL_PARAM_HEADER) : Collections.emptySet();
+    }
+
     @Override
     public String getName() {
         return "sql_query";