[SPARK-47684][SQL] Postgres: Map length unspecified bpchar to StringType

### What changes were proposed in this pull request?

This PR maps length unspecified bpchar to StringType for Postgres. Length unspecified bpchar represents variable unlimited character string, and the PG JDBC reports its size with Int.MaxValue. W/o this patch, we will always do char padding to 2147483647. This is inefficient and risky.

### Why are the changes needed?

suitable type mapping

### Does this PR introduce _any_ user-facing change?

yes, length unspecified bpchar is mapped to StringType, before, CharType(2147483647)

### How was this patch tested?

new tests

### Was this patch authored or co-authored using generative AI tooling?
no

Closes apache#45810 from yaooqinn/SPARK-47684.

Authored-by: Kent Yao <[email protected]>
Signed-off-by: Kent Yao <[email protected]>

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

@@ -122,18 +122,19 @@ class PostgresIntegrationSuite extends DockerJDBCIntegrationSuite {
     ).executeUpdate()
 
     conn.prepareStatement("CREATE TABLE char_types (" +
-      "c0 char(4), c1 character(4), c2 character varying(4), c3 varchar(4), c4 bpchar(1))"
+      "c0 char(4), c1 character(4), c2 character varying(4), c3 varchar(4), c4 bpchar(1)," +
+      "c5 bpchar, c6 char)"
     ).executeUpdate()
     conn.prepareStatement("INSERT INTO char_types VALUES " +
-      "('abcd', 'efgh', 'ijkl', 'mnop', 'q')").executeUpdate()
+      "('abcd', 'efgh', 'ijkl', 'mnop', 'q', 'eason', 'c' )").executeUpdate()
 
-    // SPARK-42916: character/char/bpchar w/o length specifier defaults to int max value, this will
-    // cause OOM as it will be padded with ' ' to 2147483647.
     conn.prepareStatement("CREATE TABLE char_array_types (" +
-      "c0 char(4)[], c1 character(4)[], c2 character varying(4)[], c3 varchar(4)[], c4 bpchar(1)[])"
+      "c0 char(4)[], c1 character(4)[], c2 character varying(4)[], c3 varchar(4)[]," +
+      "c4 bpchar(1)[], c5 bpchar[])"
     ).executeUpdate()
     conn.prepareStatement("INSERT INTO char_array_types VALUES " +
-      """('{"a", "bcd"}', '{"ef", "gh"}', '{"i", "j", "kl"}', '{"mnop"}', '{"q", "r"}')"""
+      """('{"a", "bcd"}', '{"ef", "gh"}', '{"i", "j", "kl"}', '{"mnop"}', '{"q", "r"}',
+        | '{"Eason", "Ethan"}')""".stripMargin
     ).executeUpdate()
 
     conn.prepareStatement("CREATE TABLE money_types (" +
@@ -184,7 +185,7 @@ 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 == 42)
+    assert(types.length == 45)
     assert(classOf[String].isAssignableFrom(types(0)))
     assert(classOf[java.lang.Integer].isAssignableFrom(types(1)))
     assert(classOf[java.lang.Double].isAssignableFrom(types(2)))
@@ -387,26 +388,13 @@ class PostgresIntegrationSuite extends DockerJDBCIntegrationSuite {
 
   test("character type tests") {
     val df = sqlContext.read.jdbc(jdbcUrl, "char_types", new Properties)
-    val row = df.collect()
-    assert(row.length == 1)
-    assert(row(0).length === 5)
-    assert(row(0).getString(0) === "abcd")
-    assert(row(0).getString(1) === "efgh")
-    assert(row(0).getString(2) === "ijkl")
-    assert(row(0).getString(3) === "mnop")
-    assert(row(0).getString(4) === "q")
+    checkAnswer(df, Row("abcd", "efgh", "ijkl", "mnop", "q", "eason", "c"))
   }
 
   test("SPARK-32576: character array type tests") {
     val df = sqlContext.read.jdbc(jdbcUrl, "char_array_types", new Properties)
-    val row = df.collect()
-    assert(row.length == 1)
-    assert(row(0).length === 5)
-    assert(row(0).getSeq[String](0) === Seq("a   ", "bcd "))
-    assert(row(0).getSeq[String](1) === Seq("ef  ", "gh  "))
-    assert(row(0).getSeq[String](2) === Seq("i", "j", "kl"))
-    assert(row(0).getSeq[String](3) === Seq("mnop"))
-    assert(row(0).getSeq[String](4) === Seq("q", "r"))
+    checkAnswer(df, Row(Seq("a   ", "bcd "), Seq("ef  ", "gh  "), Seq("i", "j", "kl"),
+      Seq("mnop"), Seq("q", "r"), Seq("Eason", "Ethan")))
   }
 
   test("SPARK-34333: money type tests") {

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

@@ -66,6 +66,9 @@ private case class PostgresDialect() extends JdbcDialect with SQLConfHelper {
         // timetz represents time with time zone, currently it maps to Types.TIME.
         // We need to change to Types.TIME_WITH_TIMEZONE if the upstream changes.
         Some(TimestampType)
+      case Types.CHAR if "bpchar".equalsIgnoreCase(typeName) && size == Int.MaxValue =>
+        // bpchar with unspecified length same as text in postgres with blank-trimmed
+        Some(StringType)
       case Types.OTHER if "void".equalsIgnoreCase(typeName) => Some(NullType)
       case Types.OTHER => Some(StringType)
       case _ if "text".equalsIgnoreCase(typeName) => Some(StringType) // sqlType is Types.VARCHAR
@@ -91,6 +94,7 @@ private case class PostgresDialect() extends JdbcDialect with SQLConfHelper {
     case "float4" => Some(FloatType)
     case "float8" => Some(DoubleType)
     case "varchar" => Some(VarcharType(precision))
+    case "bpchar" if precision == Int.MaxValue => Some(StringType)
     case "char" | "bpchar" => Some(CharType(precision))
     case "text" | "cidr" | "inet" | "json" | "jsonb" | "uuid" |
          "xml" | "tsvector" | "tsquery" | "macaddr" | "macaddr8" | "txid_snapshot" | "point" |