Fix jdbc-v2: report precision/scale of SimpleAggregateFunction-wrapped columns

CHANGELOG.md

@@ -101,6 +101,13 @@
 
 ### Bug Fixes 
 
+- **[jdbc-v2]** Fixed `ResultSetMetaData.getPrecision()` and `getScale()` returning `0` for columns wrapped in
+  `SimpleAggregateFunction(func, T)`. The wrapper is transparent on the read path (values are read as plain `T`), but
+  both accessors described the wrapper itself, which carries no precision or scale — so a
+  `SimpleAggregateFunction(sum, Decimal(18, 4))` column looked like a scale-0 value and
+  `SimpleAggregateFunction(any, DateTime64(3, tz))` looked like second precision. They now describe the nested type.
+  `AggregateFunction` columns are unchanged, since their values are aggregation states rather than values of the
+  nested type. (https://github.com/ClickHouse/clickhouse-java/issues/3042)
 - **[clickhouse-jdbc]** Fixed `Connection#prepareStatement` throwing a `NullPointerException` for an
   `INSERT ... VALUES (...)` statement whose values list the JavaCC parser cannot parse — most commonly one
   containing a heredoc string (`$$...$$`), which the grammar has no token for, but also any other unparsable token

jdbc-v2/src/main/java/com/clickhouse/jdbc/metadata/ResultSetMetaDataImpl.java

@@ -239,12 +239,27 @@ public String getSchemaName(int column) throws SQLException {
 
     @Override
     public int getPrecision(int column) throws SQLException {
-        return getColumn(column).getPrecision();
+        return valueColumn(getColumn(column)).getPrecision();
     }
 
     @Override
     public int getScale(int column) throws SQLException {
-        return getColumn(column).getScale();
+        return valueColumn(getColumn(column)).getScale();
+    }
+
+    /**
+     * Returns the column describing the values a caller actually reads. {@code SimpleAggregateFunction(func, T)}
+     * stores and returns plain {@code T} values, so precision and scale are declared by the nested column - the
+     * wrapper itself carries none. {@code AggregateFunction} is not transparent (its values are aggregation states)
+     * and is therefore returned as is.
+     */
+    private static ClickHouseColumn valueColumn(ClickHouseColumn column) {
+        ClickHouseColumn current = column;
+        while (current.getDataType() == ClickHouseDataType.SimpleAggregateFunction
+                && !current.getNestedColumns().isEmpty()) {
+            current = current.getNestedColumns().get(0);
+        }
+        return current;
     }
 
     @Override

jdbc-v2/src/test/java/com/clickhouse/jdbc/metadata/ResultSetMetaDataImplTest.java

@@ -2,6 +2,7 @@
 
 import com.clickhouse.jdbc.JdbcIntegrationTest;
 import org.testng.Assert;
+import org.testng.annotations.DataProvider;
 import org.testng.annotations.Test;
 
 import java.sql.Connection;
@@ -237,6 +238,36 @@ public void testGetColumnScale() throws Exception {
         }
     }
 
+    @DataProvider(name = "wrappedTypePrecisionAndScale")
+    public static Object[][] wrappedTypePrecisionAndScaleProvider() {
+        return new Object[][] {
+                { "CAST(toDecimal64(1, 4) AS Decimal(18, 4))", 18, 4 },
+                { "CAST(toDecimal64(1, 4) AS SimpleAggregateFunction(any, Decimal(18, 4)))", 18, 4 },
+                { "CAST(toDecimal128(1, 10) AS SimpleAggregateFunction(sum, Decimal(38, 10)))", 38, 10 },
+                { "CAST(NULL AS SimpleAggregateFunction(any, Nullable(Decimal(18, 4))))", 18, 4 },
+                { "CAST(now() AS SimpleAggregateFunction(any, DateTime('Europe/Amsterdam')))", 29, 0 },
+                { "CAST(now64(3, 'Europe/Amsterdam') AS "
+                        + "SimpleAggregateFunction(any, DateTime64(3, 'Europe/Amsterdam')))", 29, 3 },
+                { "CAST('abc' AS SimpleAggregateFunction(any, FixedString(16)))", 16, 0 },
+                { "CAST([toDecimal64(1, 4)] AS "
+                        + "SimpleAggregateFunction(groupArrayArray, Array(Decimal(18, 4))))", 0, 0 },
+                { "toLowCardinality(toDateTime('2024-01-15 12:30:45', 'Europe/Amsterdam'))", 29, 0 },
+                { "sumState(toDecimal64(1, 4))", 0, 0 },
+        };
+    }
+
+    @Test(groups = { "integration" }, dataProvider = "wrappedTypePrecisionAndScale")
+    public void testGetPrecisionAndScaleOfWrappedTypes(String expression, int precision, int scale) throws Exception {
+        try (Connection conn = getJdbcConnection();
+             Statement stmt = conn.createStatement();
+             ResultSet rs = stmt.executeQuery("SELECT " + expression + " AS v")) {
+            ResultSetMetaData rsmd = rs.getMetaData();
+            String type = rsmd.getColumnTypeName(1);
+            assertEquals(rsmd.getPrecision(1), precision, type);
+            assertEquals(rsmd.getScale(1), scale, type);
+        }
+    }
+
     @Test(groups = { "integration" })
     public void testColumnNamesStrippedFromTablePrefix() throws Exception {
         final String t1 = "rsmd_test_prefix_t1";