MySQL compat: Align behavior of %f in formatDateTime() with parseDateTime()

docs/en/sql-reference/functions/date-time-functions.md

@@ -1264,7 +1264,7 @@ Using replacement fields, you can define a pattern for the resulting string. “
 | %d       | day of the month, zero-padded (01-31)                   | 02         |
 | %D       | Short MM/DD/YY date, equivalent to %m/%d/%y             | 01/02/18   |
 | %e       | day of the month, space-padded (1-31)                   |   2   |
-| %f       | fractional second from the fractional part of DateTime64 | 1234560   |
+| %f       | fractional second, see below (*)                        | 1234560   |
 | %F       | short YYYY-MM-DD date, equivalent to %Y-%m-%d           | 2018-01-02 |
 | %g       | two-digit year format, aligned to ISO 8601, abbreviated from four-digit notation                                | 18       |
 | %G       | four-digit year format for ISO week number, calculated from the week-based year [defined by the ISO 8601](https://en.wikipedia.org/wiki/ISO_8601#Week_dates) standard, normally useful only with %V  | 2018         |
@@ -1295,6 +1295,8 @@ Using replacement fields, you can define a pattern for the resulting string. “
 | %z       | Time offset from UTC as +HHMM or -HHMM                  | -0500      |
 | %%       | a % sign                                                | %          |
 
+(*) The behavior of `%f` is to print `000000` (six zeros) if the formatted value is a Date, Date32 or DateTime (which have no fractional seconds) or a DateTime64 with a precision of 0. Earlier versions of ClickHouse printed `0` in this case. The previous behavior can be restored using setting `formatdatetime_f_prints_single_zero = 1`.
+
 **Example**
 
 Query:

src/Core/Settings.h

@@ -467,7 +467,8 @@ class IColumn;
     M(Bool, allow_introspection_functions, false, "Allow functions for introspection of ELF and DWARF for query profiling. These functions are slow and may impose security considerations.", 0) \
     \
     M(Bool, allow_execute_multiif_columnar, true, "Allow execute multiIf function columnar", 0) \
-    M(Bool, formatdatetime_parsedatetime_m_is_month_name, true, "Formatter '%M' in function 'formatDateTime' produces the month name instead of minutes.", 0) \
+    M(Bool, formatdatetime_f_prints_single_zero, false, "Formatter '%f' in function 'formatDateTime()' produces a single zero instead of six zeros if the formatted value has no fractional seconds.", 0) \
+    M(Bool, formatdatetime_parsedatetime_m_is_month_name, true, "Formatter '%M' in functions 'formatDateTime()' and 'parseDateTime()' produces the month name instead of minutes.", 0) \
     \
     M(UInt64, max_partitions_per_insert_block, 100, "Limit maximum number of partitions in single INSERTed block. Zero means unlimited. Throw exception if the block contains too many partitions. This setting is a safety threshold, because using large number of partitions is a common misconception.", 0) \
     M(Int64, max_partitions_to_read, -1, "Limit the max number of partitions that can be accessed in one query. <= 0 means unlimited.", 0) \

src/Core/SettingsChangesHistory.h

@@ -101,6 +101,7 @@ static std::map<ClickHouseVersion, SettingsChangesHistory::SettingsChanges> sett
                {"query_plan_aggregation_in_order", 0, 1, "Enable some refactoring around query plan"},
                {"format_binary_max_string_size", 0, 1_GiB, "Prevent allocating large amount of memory"}}},
     {"22.11", {{"use_structure_from_insertion_table_in_table_functions", 0, 2, "Improve using structure from insertion table in table functions"}}},
+    {"23.4", {{"formatdatetime_f_prints_single_zero", true, false, "Improved compatibility with MySQL DATE_FORMAT()/STR_TO_DATE()"}}},
     {"23.4", {{"formatdatetime_parsedatetime_m_is_month_name", false, true, "Improved compatibility with MySQL DATE_FORMAT/STR_TO_DATE"}}},
     {"22.9", {{"force_grouping_standard_compatibility", false, true, "Make GROUPING function output the same as in SQL standard and other DBMS"}}},
     {"22.7", {{"cross_to_inner_join_rewrite", 1, 2, "Force rewrite comma join to inner"},

src/Functions/formatDateTime.cpp

@@ -449,6 +449,20 @@ private:
         }
 
         size_t mysqlFractionalSecond(char * dest, Time /*source*/, UInt64 fractional_second, UInt32 scale, const DateLUTImpl & /*timezone*/)
+        {
+            if (scale == 0)
+                scale = 6;
+
+            for (Int64 i = scale, value = fractional_second; i > 0; --i)
+            {
+                dest[i - 1] += value % 10;
+                value /= 10;
+            }
+            return scale;
+        }
+
+        /// Same as mysqlFractionalSecond but prints a single zero if the value has no fractional seconds
+        size_t mysqlFractionalSecondSingleZero(char * dest, Time /*source*/, UInt64 fractional_second, UInt32 scale, const DateLUTImpl & /*timezone*/)
         {
             if (scale == 0)
                 scale = 1;
@@ -710,6 +724,7 @@ private:
     }
 
     const bool mysql_M_is_month_name;
+    const bool mysql_f_prints_single_zero;
 
 public:
     static constexpr auto name = Name::name;
@@ -718,6 +733,7 @@ public:
 
     explicit FunctionFormatDateTimeImpl(ContextPtr context)
         : mysql_M_is_month_name(context->getSettings().formatdatetime_parsedatetime_m_is_month_name)
+        , mysql_f_prints_single_zero(context->getSettings().formatdatetime_f_prints_single_zero)
     {
     }
 
@@ -1116,11 +1132,21 @@ public:
                     // Fractional seconds
                     case 'f':
                     {
-                        /// If the time data type has no fractional part, then we print '0' as the fractional part.
+                        /// If the time data type has no fractional part, we print (default) '000000' or (deprecated) '0' as fractional part.
+                        if (mysql_f_prints_single_zero)
+                        {
+                            Instruction<T> instruction;
+                            instruction.setMysqlFunc(&Instruction<T>::mysqlFractionalSecondSingleZero);
+                            instructions.push_back(std::move(instruction));
+                            out_template += String(scale == 0 ? 1 : scale, '0');
+                        }
+                        else
+                        {
                             Instruction<T> instruction;
                             instruction.setMysqlFunc(&Instruction<T>::mysqlFractionalSecond);
                             instructions.push_back(std::move(instruction));
-                        out_template += String(std::max<UInt32>(1, scale), '0');
+                            out_template += String(scale == 0 ? 6 : scale, '0');
+                        }
                         break;
                     }
 

tests/queries/0_stateless/00718_format_datetime.reference

@@ -5,6 +5,7 @@ Jan	Jan
 02	02
 01/02/18	01/02/18
  2	 2
+000000	000000
 2018-01-02	2018-01-02
 10	12
 22	00
@@ -43,11 +44,23 @@ no formatting pattern	no formatting pattern
 -1100
 +0300
 +0530
-1234560
+000000
-000340
+000000
+000000
+000000
+123
+123456
+123456789
+0
+0
+0
+0
+123
+123456
+123456789
 2022-12-08 18:11:29.123400000
 2022-12-08 18:11:29.1
-2022-12-08 18:11:29.0
+2022-12-08 18:11:29.000000
-2022-12-08 18:11:29.0
+2022-12-08 18:11:29.000000
-2022-12-08 00:00:00.0
+2022-12-08 00:00:00.000000
-2022-12-08 00:00:00.0
+2022-12-08 00:00:00.000000

tests/queries/0_stateless/00718_format_datetime.sql

@@ -17,6 +17,7 @@ SELECT formatDateTime(toDateTime('2018-01-02 22:33:44'), '%C'), formatDateTime(t
 SELECT formatDateTime(toDateTime('2018-01-02 22:33:44'), '%d'), formatDateTime(toDate32('2018-01-02'), '%d');
 SELECT formatDateTime(toDateTime('2018-01-02 22:33:44'), '%D'), formatDateTime(toDate32('2018-01-02'), '%D');
 SELECT formatDateTime(toDateTime('2018-01-02 22:33:44'), '%e'), formatDateTime(toDate32('2018-01-02'), '%e');
+SELECT formatDateTime(toDateTime('2018-01-02 22:33:44'), '%f'), formatDateTime(toDate32('2018-01-02'), '%f');
 SELECT formatDateTime(toDateTime('2018-01-02 22:33:44'), '%F'), formatDateTime(toDate32('2018-01-02'), '%F');
 SELECT formatDateTime(toDateTime('2018-01-02 22:33:44'), '%h'), formatDateTime(toDate32('2018-01-02'), '%h');
 SELECT formatDateTime(toDateTime('2018-01-02 22:33:44'), '%H'), formatDateTime(toDate32('2018-01-02'), '%H');
@@ -66,8 +67,22 @@ SELECT formatDateTime(toDateTime('2020-01-01 01:00:00', 'US/Samoa'), '%z');
 SELECT formatDateTime(toDateTime('2020-01-01 01:00:00', 'Europe/Moscow'), '%z');
 SELECT formatDateTime(toDateTime('1970-01-01 00:00:00', 'Asia/Kolkata'), '%z');
 
-select formatDateTime(toDateTime64('2010-01-04 12:34:56.123456', 7), '%f');
+-- %f (default settings)
-select formatDateTime(toDateTime64('2022-12-08 18:11:29.00034', 6, 'UTC'), '%f');
+select formatDateTime(toDate('2010-01-04'), '%f') SETTINGS formatdatetime_f_prints_single_zero = 0;
+select formatDateTime(toDate32('2010-01-04'), '%f') SETTINGS formatdatetime_f_prints_single_zero = 0;
+select formatDateTime(toDateTime('2010-01-04 12:34:56'), '%f') SETTINGS formatdatetime_f_prints_single_zero = 0;
+select formatDateTime(toDateTime64('2010-01-04 12:34:56', 0), '%f') SETTINGS formatdatetime_f_prints_single_zero = 0;
+select formatDateTime(toDateTime64('2010-01-04 12:34:56.123', 3), '%f') SETTINGS formatdatetime_f_prints_single_zero = 0;
+select formatDateTime(toDateTime64('2010-01-04 12:34:56.123456', 6), '%f') SETTINGS formatdatetime_f_prints_single_zero = 0;
+select formatDateTime(toDateTime64('2010-01-04 12:34:56.123456789', 9), '%f') SETTINGS formatdatetime_f_prints_single_zero = 0;
+-- %f (legacy settings)
+select formatDateTime(toDate('2010-01-04'), '%f') SETTINGS formatdatetime_f_prints_single_zero = 1;
+select formatDateTime(toDate32('2010-01-04'), '%f') SETTINGS formatdatetime_f_prints_single_zero = 1;
+select formatDateTime(toDateTime('2010-01-04 12:34:56'), '%f') SETTINGS formatdatetime_f_prints_single_zero = 1;
+select formatDateTime(toDateTime64('2010-01-04 12:34:56', 0), '%f') SETTINGS formatdatetime_f_prints_single_zero = 1;
+select formatDateTime(toDateTime64('2010-01-04 12:34:56.123', 3), '%f') SETTINGS formatdatetime_f_prints_single_zero = 1;
+select formatDateTime(toDateTime64('2010-01-04 12:34:56.123456', 6), '%f') SETTINGS formatdatetime_f_prints_single_zero = 0;
+select formatDateTime(toDateTime64('2010-01-04 12:34:56.123456789', 9), '%f') SETTINGS formatdatetime_f_prints_single_zero = 1;
 
 select formatDateTime(toDateTime64('2022-12-08 18:11:29.1234', 9, 'UTC'), '%F %T.%f');
 select formatDateTime(toDateTime64('2022-12-08 18:11:29.1234', 1, 'UTC'), '%F %T.%f');