Merge pull request ClickHouse#73354 from ClickHouse/backport/24.3/71396

Backport ClickHouse#71396 to 24.3: Fix: ERROR: column "attgenerated" does not exist for old PostgreSQL

src/Databases/PostgreSQL/fetchPostgreSQLTableStructure.cpp

@@ -301,18 +301,26 @@ PostgreSQLTableStructure fetchPostgreSQLTableStructure(
         ? " AND relnamespace = (SELECT oid FROM pg_namespace WHERE nspname = 'public')"
         : fmt::format(" AND relnamespace = (SELECT oid FROM pg_namespace WHERE nspname = {})", quoteString(postgres_schema));
 
+
+    /// Bypassing the error of the missing column `attgenerated` in the system table `pg_attribute` for PostgreSQL versions below 12.
+    /// This trick involves executing a special query to the DBMS in advance to obtain the correct line with comment /// if column has GENERATED.
+    /// The result of the query will be the name of the column `attgenerated` or an empty string declaration for PostgreSQL version 11 and below.
+    /// This change does not degrade the function's performance but restores support for older versions and fix ERROR: column "attgenerated" does not exist.
+    pqxx::result gen_result{tx.exec("select case when current_setting('server_version_num')::int < 120000 then '''''' else 'attgenerated' end as generated")};
+    std::string generated = gen_result[0][0].as<std::string>();
+
     std::string query = fmt::format(
            "SELECT attname AS name, " /// column name
            "format_type(atttypid, atttypmod) AS type, " /// data type
            "attnotnull AS not_null, " /// is nullable
            "attndims AS dims, " /// array dimensions
            "atttypid as type_id, "
            "atttypmod as type_modifier, "
-           "attgenerated as generated " /// if column has GENERATED
+           "{} as generated " /// if column has GENERATED
            "FROM pg_attribute "
            "WHERE attrelid = (SELECT oid FROM pg_class WHERE {}) "
            "AND NOT attisdropped AND attnum > 0 "
-           "ORDER BY attnum ASC", where);
+           "ORDER BY attnum ASC", generated, where);
 
     auto postgres_table_with_schema = postgres_schema.empty() ? postgres_table : doubleQuoteString(postgres_schema) + '.' + doubleQuoteString(postgres_table);
     table.physical_columns = readNamesAndTypesList(tx, postgres_table_with_schema, query, use_nulls, false);