sql: add the getdatabaseencoding() builtin function

Resolves cockroachdb#41771.

This commit adds builtin function, getdatabaseencoding(),
and the unit-test case for it.

Release note (sql change): This PR is introduced to add builtin
function, getdatabaseencoding(), which returns the current encoding
name used by the database.

docs/generated/sql/functions.md

@@ -1123,6 +1123,8 @@ SELECT * FROM crdb_internal.check_consistency(true, ‘\x02’, ‘\x04’)</p>
 </span></td></tr>
 <tr><td><a name="current_user"></a><code>current_user() &rarr; <a href="string.html">string</a></code></td><td><span class="funcdesc"><p>Returns the current user. This function is provided for compatibility with PostgreSQL.</p>
 </span></td></tr>
+<tr><td><a name="getdatabaseencoding"></a><code>getdatabaseencoding() &rarr; <a href="string.html">string</a></code></td><td><span class="funcdesc"><p>Returns the current encoding name used by the database.</p>
+</span></td></tr>
 <tr><td><a name="version"></a><code>version() &rarr; <a href="string.html">string</a></code></td><td><span class="funcdesc"><p>Returns the node’s version of CockroachDB.</p>
 </span></td></tr></tbody>
 </table>

pkg/sql/logictest/testdata/logic_test/builtin_function

@@ -2556,3 +2556,8 @@ SELECT timezone('1970-01-01 01:00'::timestamptz, 'UTC+6')
 
 statement ok
 SET TIME ZONE +0
+
+query T
+SELECT getdatabaseencoding()
+----
+UTF8

pkg/sql/sem/builtins/builtins.go

@@ -3064,6 +3064,28 @@ may increase either contention or retry errors, or both.`,
 		},
 	),
 
+	"getdatabaseencoding": makeBuiltin(
+		tree.FunctionProperties{Category: categorySystemInfo},
+		tree.Overload{
+			Types:      tree.ArgTypes{},
+			ReturnType: tree.FixedReturnType(types.String),
+			Fn: func(ctx *tree.EvalContext, args tree.Datums) (tree.Datum, error) {
+				// computing encoding name bypassing encoding_id which is
+				// extracted from pg_database to pg_encoding_to_char function.
+				row, err := ctx.InternalExecutor.QueryRow(
+					ctx.Ctx(), "getdatabaseencoding",
+					ctx.Txn,
+					"SELECT pg_encoding_to_char(encoding) "+
+						"FROM pg_database WHERE datname = $1", ctx.SessionData.Database)
+				if err != nil {
+					return nil, err
+				}
+				return row[0], nil
+			},
+			Info: "Returns the current encoding name used by the database.",
+		},
+	),
+
 	// https://www.postgresql.org/docs/10/static/functions-info.html
 	//
 	// Note that in addition to what the pg doc says ("current_schema =