Add Flint describe index SQL support

flint/flint-spark-integration/src/main/antlr4/FlintSparkSqlExtensions.g4

@@ -19,7 +19,12 @@ statement
     ;
 
 skippingIndexStatement
-    : dropSkippingIndexStatement
+    : describeSkippingIndexStatement
+    | dropSkippingIndexStatement
+    ;
+
+describeSkippingIndexStatement
+    : (DESC | DESCRIBE) SKIPPING INDEX ON tableName=multipartIdentifier
     ;
 
 dropSkippingIndexStatement

flint/flint-spark-integration/src/main/antlr4/SparkSqlBase.g4

@@ -113,6 +113,8 @@ SKIPPING : 'SKIPPING';
 
 SEMICOLON: ';';
 
+DESC: 'DESC';
+DESCRIBE: 'DESCRIBE';
 DOT: '.';
 DROP: 'DROP';
 INDEX: 'INDEX';

flint/flint-spark-integration/src/main/scala/org/opensearch/flint/spark/sql/FlintSparkSqlAstBuilder.scala

@@ -5,26 +5,47 @@
 
 package org.opensearch.flint.spark.sql
 
+import org.opensearch.flint.spark.skipping.FlintSparkSkippingIndex
 import org.opensearch.flint.spark.skipping.FlintSparkSkippingIndex.getSkippingIndexName
-import org.opensearch.flint.spark.sql.FlintSparkSqlExtensionsParser.DropSkippingIndexStatementContext
+import org.opensearch.flint.spark.sql.FlintSparkSqlExtensionsParser.{DescribeSkippingIndexStatementContext, DropSkippingIndexStatementContext}
 
+import org.apache.spark.sql.Row
+import org.apache.spark.sql.catalyst.expressions.AttributeReference
 import org.apache.spark.sql.catalyst.plans.logical.Command
+import org.apache.spark.sql.types.StringType
 
 /**
  * Flint Spark AST builder that builds Spark command for Flint index statement.
  */
 class FlintSparkSqlAstBuilder extends FlintSparkSqlExtensionsBaseVisitor[Command] {
 
-  override def visitDropSkippingIndexStatement(
-      ctx: DropSkippingIndexStatementContext): Command = {
-    FlintSparkSqlCommand { flint =>
+  override def visitDescribeSkippingIndexStatement(
+      ctx: DescribeSkippingIndexStatementContext): Command = {
+    val outputSchema = Seq(
+      AttributeReference("indexed_col_name", StringType, nullable = false)(),
+      AttributeReference("data_type", StringType, nullable = false)(),
+      AttributeReference("skip_type", StringType, nullable = false)())
+
+    FlintSparkSqlCommand(outputSchema) { flint =>
+      val indexName = getSkippingIndexName(ctx.tableName.getText)
+      flint
+        .describeIndex(indexName)
+        .map { case index: FlintSparkSkippingIndex =>
+          index.indexedColumns.map(strategy =>
+            Row(strategy.columnName, strategy.columnType, strategy.kind.toString))
+        }
+        .getOrElse(Seq.empty)
+    }
+  }
+
+  override def visitDropSkippingIndexStatement(ctx: DropSkippingIndexStatementContext): Command =
+    FlintSparkSqlCommand() { flint =>
       val tableName = ctx.tableName.getText // TODO: handle schema name
       val indexName = getSkippingIndexName(tableName)
       flint.deleteIndex(indexName)
       Seq.empty
     }
-  }
 
   override def aggregateResult(aggregate: Command, nextResult: Command): Command =
-    if (nextResult != null) nextResult else aggregate;
+    if (nextResult != null) nextResult else aggregate
 }

flint/flint-spark-integration/src/main/scala/org/opensearch/flint/spark/sql/FlintSparkSqlCommand.scala

@@ -8,6 +8,7 @@ package org.opensearch.flint.spark.sql
 import org.opensearch.flint.spark.FlintSpark
 
 import org.apache.spark.sql.{Row, SparkSession}
+import org.apache.spark.sql.catalyst.expressions.Attribute
 import org.apache.spark.sql.execution.command.LeafRunnableCommand
 
 /**
@@ -19,7 +20,12 @@ import org.apache.spark.sql.execution.command.LeafRunnableCommand
  * @param block
  *   code block that triggers Flint core API
  */
-case class FlintSparkSqlCommand(block: FlintSpark => Seq[Row]) extends LeafRunnableCommand {
+case class FlintSparkSqlCommand(override val output: Seq[Attribute] = Seq.empty)(
+    block: FlintSpark => Seq[Row])
+    extends LeafRunnableCommand {
 
   override def run(sparkSession: SparkSession): Seq[Row] = block(new FlintSpark(sparkSession))
+
+  // Lazy arguments are required to specify here
+  override protected def otherCopyArgs: Seq[AnyRef] = block :: Nil
 }

flint/integ-test/src/test/scala/org/opensearch/flint/spark/FlintSparkSqlSuite.scala

@@ -12,7 +12,7 @@ import org.opensearch.flint.spark.skipping.FlintSparkSkippingIndex.getSkippingIn
 import org.scalatest.matchers.should.Matchers.convertToAnyShouldWrapper
 
 import org.apache.spark.FlintSuite
-import org.apache.spark.sql.QueryTest
+import org.apache.spark.sql.{QueryTest, Row}
 import org.apache.spark.sql.flint.config.FlintSparkConf.{HOST_ENDPOINT, HOST_PORT}
 
 class FlintSparkSqlSuite extends QueryTest with FlintSuite with OpenSearchSuite {
@@ -25,16 +25,16 @@ class FlintSparkSqlSuite extends QueryTest with FlintSuite with OpenSearchSuite
   }
 
   /** Test table and index name */
-  private val testTable = "test"
+  private val testTable = "flint_sql_test"
   private val testIndex = getSkippingIndexName(testTable)
 
   override def beforeAll(): Unit = {
     super.beforeAll()
-
     sql(s"""
            | CREATE TABLE $testTable
            | (
-           |   name STRING
+           |   name STRING,
+           |   age INT
            | )
            | USING CSV
            | OPTIONS (
@@ -48,13 +48,41 @@ class FlintSparkSqlSuite extends QueryTest with FlintSuite with OpenSearchSuite
            |""".stripMargin)
   }
 
-  test("drop skipping index") {
+  protected override def beforeEach(): Unit = {
+    super.beforeEach()
     flint
       .skippingIndex()
       .onTable(testTable)
       .addPartitions("year")
+      .addValueSet("name")
+      .addMinMax("age")
       .create()
+  }
+
+  protected override def afterEach(): Unit = {
+    super.afterEach()
+    flint.deleteIndex(testIndex)
+  }
+
+  test("describe skipping index") {
+    val result = sql(s"DESC SKIPPING INDEX ON $testTable")
 
+    checkAnswer(
+      result,
+      Seq(
+        Row("year", "int", "Partition"),
+        Row("name", "string", "ValuesSet"),
+        Row("age", "int", "MinMax")))
+  }
+
+  test("should return empty if no skipping index to describe") {
+    flint.deleteIndex(testIndex)
+
+    val result = sql(s"DESC SKIPPING INDEX ON $testTable")
+    checkAnswer(result, Seq.empty)
+  }
+
+  test("drop skipping index") {
     sql(s"DROP SKIPPING INDEX ON $testTable")
 
     flint.describeIndex(testIndex) shouldBe empty