Fix several issues regarding PostgreSQL array_ndims usage.

1. Properly quote column identifiers.
2. Handle empty tables that have array columns.
3. Throw a more user friendly error when column value = NULL
   when calling array_dims(column)
4. Handle column value being the empty array {}

src/Databases/PostgreSQL/fetchPostgreSQLTableStructure.cpp

@@ -158,6 +158,17 @@ static DataTypePtr convertPostgreSQLDataType(String & type, Fn<void()> auto && r
     return res;
 }
 
+/// Check if PostgreSQL relation is empty.
+/// postgres_table must be already quoted + schema-qualified.
+template<typename T>
+bool isTableEmpty(T &tx, const String & postgres_table) {
+  auto query = fmt::format(
+    "SELECT NOT EXISTS (SELECT * FROM {} LIMIT 1);",
+    postgres_table
+  );
+  pqxx::result result{tx.exec(query)};
+  return result[0][0].as<bool>();
+}
 
 template<typename T>
 PostgreSQLTableStructure::ColumnsInfoPtr readNamesAndTypesList(
@@ -213,10 +224,39 @@ PostgreSQLTableStructure::ColumnsInfoPtr readNamesAndTypesList(
         {
             const auto & name_and_type = columns[i];
 
-            /// All rows must contain the same number of dimensions, so limit 1 is ok. If number of dimensions in all rows is not the same -
+            /// NOTE: If the relation is empty, then array_ndims returns NULL.
-            /// such arrays are not able to be used as ClickHouse Array at all.
+            /// If this is the case, then assume dimensions=1. This covers most
-            pqxx::result result{tx.exec(fmt::format("SELECT array_ndims({}) FROM {} LIMIT 1", name_and_type.name, postgres_table))};
+            /// use cases, but will be incorrect for empty tables with
-            auto dimensions = result[0][0].as<int>();
+            /// multi-dimension arrays. The other solutions would be to drop
+            /// support for empty tables OR attempt fallback to a discovered
+            /// array_ndims CHECK constraint.
+            int dimensions;
+            if (isTableEmpty(tx, postgres_table)) {
+              dimensions = 1;
+            } else {
+              /// All rows must contain the same number of dimensions.
+              /// 1 is ok. If number of dimensions in all rows is not the same -
+              /// such arrays are not able to be used as ClickHouse Array at all.
+              ///
+              /// Assume dimensions=1 for empty arrays.
+              auto postgres_column = doubleQuoteString(name_and_type.name);
+              pqxx::result result{tx.exec(fmt::format(
+                  "SELECT {} IS NULL, COALESCE(array_ndims({}), 1) "
+                  "FROM {} LIMIT 1;",
+                  postgres_column, postgres_column, postgres_table
+              ))};
+
+              /// Nullable(Array) is not supported.
+              auto is_null = result[0][0].as<bool>();
+              if (is_null) {
+                throw Exception(
+                    ErrorCodes::BAD_ARGUMENTS,
+                    "PostgreSQL array cannot be NULL. Column: {}", postgres_column
+                );
+              }
+
+              dimensions = result[0][1].as<int>();
+            }
 
             /// It is always 1d array if it is in recheck.
             DataTypePtr type = assert_cast<const DataTypeArray *>(name_and_type.type.get())->getNestedType();