[SPARK-26538][SQL] Set default precision and scale for elements of postgres numeric array

## What changes were proposed in this pull request?

When determining CatalystType for postgres columns with type `numeric[]` set the type of array element to `DecimalType(38, 18)` instead of `DecimalType(0,0)`.

## How was this patch tested?

Tested with modified `org.apache.spark.sql.jdbc.JDBCSuite`.
Ran the `PostgresIntegrationSuite` manually.

Closes #23456 from a-shkarupin/postgres_numeric_array.

Lead-authored-by: Oleksii Shkarupin <a.shkarupin@gmail.com>
Co-authored-by: Dongjoon Hyun <dongjoon@apache.org>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>

external/docker-integration-tests/src/test/scala/org/apache/spark/sql/jdbc/PostgresIntegrationSuite.scala

@@ -46,14 +46,15 @@ class PostgresIntegrationSuite extends DockerJDBCIntegrationSuite {
     conn.prepareStatement("CREATE TABLE bar (c0 text, c1 integer, c2 double precision, c3 bigint, "
       + "c4 bit(1), c5 bit(10), c6 bytea, c7 boolean, c8 inet, c9 cidr, "
       + "c10 integer[], c11 text[], c12 real[], c13 numeric(2,2)[], c14 enum_type, "
-      + "c15 float4, c16 smallint)").executeUpdate()
+      + "c15 float4, c16 smallint, c17 numeric[])").executeUpdate()
     conn.prepareStatement("INSERT INTO bar VALUES ('hello', 42, 1.25, 123456789012345, B'0', "
       + "B'1000100101', E'\\\\xDEADBEEF', true, '172.16.0.42', '192.168.0.0/16', "
-      + """'{1, 2}', '{"a", null, "b"}', '{0.11, 0.22}', '{0.11, 0.22}', 'd1', 1.01, 1)"""
+      + """'{1, 2}', '{"a", null, "b"}', '{0.11, 0.22}', '{0.11, 0.22}', 'd1', 1.01, 1, """
+      + "'{111.2222, 333.4444}')"
     ).executeUpdate()
     conn.prepareStatement("INSERT INTO bar VALUES (null, null, null, null, null, "
       + "null, null, null, null, null, "
-      + "null, null, null, null, null, null, null)"
+      + "null, null, null, null, null, null, null, null)"
     ).executeUpdate()
 
     conn.prepareStatement("CREATE TABLE ts_with_timezone " +
@@ -85,7 +86,7 @@ class PostgresIntegrationSuite extends DockerJDBCIntegrationSuite {
     assert(rows.length == 2)
     // Test the types, and values using the first row.
     val types = rows(0).toSeq.map(x => x.getClass)
-    assert(types.length == 17)
+    assert(types.length == 18)
     assert(classOf[String].isAssignableFrom(types(0)))
     assert(classOf[java.lang.Integer].isAssignableFrom(types(1)))
     assert(classOf[java.lang.Double].isAssignableFrom(types(2)))
@@ -103,6 +104,7 @@ class PostgresIntegrationSuite extends DockerJDBCIntegrationSuite {
     assert(classOf[String].isAssignableFrom(types(14)))
     assert(classOf[java.lang.Float].isAssignableFrom(types(15)))
     assert(classOf[java.lang.Short].isAssignableFrom(types(16)))
+    assert(classOf[Seq[BigDecimal]].isAssignableFrom(types(17)))
     assert(rows(0).getString(0).equals("hello"))
     assert(rows(0).getInt(1) == 42)
     assert(rows(0).getDouble(2) == 1.25)
@@ -123,6 +125,8 @@ class PostgresIntegrationSuite extends DockerJDBCIntegrationSuite {
     assert(rows(0).getString(14) == "d1")
     assert(rows(0).getFloat(15) == 1.01f)
     assert(rows(0).getShort(16) == 1)
+    assert(rows(0).getSeq(17) ==
+      Seq("111.222200000000000000", "333.444400000000000000").map(BigDecimal(_).bigDecimal))
 
     // Test reading null values using the second row.
     assert(0.until(16).forall(rows(1).isNullAt(_)))

sql/core/src/main/scala/org/apache/spark/sql/jdbc/PostgresDialect.scala

@@ -60,7 +60,10 @@ private object PostgresDialect extends JdbcDialect {
     case "bytea" => Some(BinaryType)
     case "timestamp" | "timestamptz" | "time" | "timetz" => Some(TimestampType)
     case "date" => Some(DateType)
-    case "numeric" | "decimal" => Some(DecimalType.bounded(precision, scale))
+    case "numeric" | "decimal" if precision > 0 => Some(DecimalType.bounded(precision, scale))
+    case "numeric" | "decimal" =>
+      // SPARK-26538: handle numeric without explicit precision and scale.
+      Some(DecimalType. SYSTEM_DEFAULT)
     case _ => None
   }
 

sql/core/src/test/scala/org/apache/spark/sql/jdbc/JDBCSuite.scala

@@ -850,8 +850,11 @@ class JDBCSuite extends QueryTest
 
   test("PostgresDialect type mapping") {
     val Postgres = JdbcDialects.get("jdbc:postgresql://127.0.0.1/db")
+    val md = new MetadataBuilder().putLong("scale", 0)
     assert(Postgres.getCatalystType(java.sql.Types.OTHER, "json", 1, null) === Some(StringType))
     assert(Postgres.getCatalystType(java.sql.Types.OTHER, "jsonb", 1, null) === Some(StringType))
+    assert(Postgres.getCatalystType(java.sql.Types.ARRAY, "_numeric", 0, md) ==
+      Some(ArrayType(DecimalType.SYSTEM_DEFAULT)))
     assert(Postgres.getJDBCType(FloatType).map(_.databaseTypeDefinition).get == "FLOAT4")
     assert(Postgres.getJDBCType(DoubleType).map(_.databaseTypeDefinition).get == "FLOAT8")
     val errMsg = intercept[IllegalArgumentException] {