Improvements to Table.union (#9968)

- Closes #9952

CHANGELOG.md

@@ -666,6 +666,7 @@
 - [Added `Text.cleanse` `Column.Text_Cleanse` and `Table.Text_Cleanse`][9879]
 - [Added ability to save an existing Postgres connection as a Data Link in Enso
   Cloud.][9957]
+- [Improved `Table.union`.][9968]
 
 [debug-shortcuts]:
   https://github.com/enso-org/enso/blob/develop/app/gui/docs/product/shortcuts.md#debug
@@ -978,6 +979,7 @@
 [9873]: https://github.com/enso-org/enso/pull/9873
 [9879]: https://github.com/enso-org/enso/pull/9879
 [9957]: https://github.com/enso-org/enso/pull/9957
+[9968]: https://github.com/enso-org/enso/pull/9968
 
 #### Enso Compiler
 

distribution/lib/Standard/Database/0.0.0-dev/src/Errors.enso

@@ -35,11 +35,18 @@ type SQL_Error
        Convert the SQL error to a textual representation.
     to_text : Text
     to_text self =
-        query = if self.related_query.is_nothing.not then " [Query was: " + self.related_query.to_display_text + "]" else ""
+        query = if self.related_query.is_nothing then "" else
+            query_text = self.related_query.to_text
+            ## Our generated queries tend to be very long, so to still be readable,
+               we don't shorten them too much. We impose an upper limit to avoid unbounded error message size.
+            max_length = 1000
+            shortened_query_text = if query_text.length <= max_length then query_text else
+                query_text.take (Index_Sub_Range.First (max_length.div 2)) + " (...) " + query_text.take (Index_Sub_Range.Last (max_length.div 2))
+            " [Query was: " + shortened_query_text + "]"
         message = self.java_exception.getMessage
         max_length = 300
         short_message = if message.length < max_length then message else
-            message.take (Index_Sub_Range.First max_length/2) + " (...) " + message.take (Index_Sub_Range.Last max_length/2)
+            message.take (Index_Sub_Range.First (max_length.div 2)) + " (...) " + message.take (Index_Sub_Range.Last (max_length.div 2))
         "There was an SQL error: " + short_message + "." + query
 
     ## PRIVATE

distribution/lib/Standard/Database/0.0.0-dev/src/Internal/Postgres/Postgres_Dialect.enso

@@ -140,7 +140,7 @@ type Postgres_Dialect
 
     ## PRIVATE
     make_cast : Internal_Column -> SQL_Type -> (SQL_Expression -> SQL_Type_Reference) -> Internal_Column
-    make_cast self column target_type infer_result_type_from_database_callback =
+    make_cast self (column : Internal_Column) (target_type : SQL_Type) (infer_result_type_from_database_callback : SQL_Expression -> SQL_Type_Reference) =
         mapping = self.get_type_mapping
         source_type = mapping.sql_type_to_value_type column.sql_type_reference.get
         target_value_type = mapping.sql_type_to_value_type target_type

distribution/lib/Standard/Database/0.0.0-dev/src/Internal/SQLite/SQLite_Dialect.enso

@@ -138,7 +138,7 @@ type SQLite_Dialect
 
     ## PRIVATE
     make_cast : Internal_Column -> SQL_Type -> (SQL_Expression -> SQL_Type_Reference) -> Internal_Column
-    make_cast self column target_type infer_result_type_from_database_callback =
+    make_cast self (column : Internal_Column) (target_type : SQL_Type) (infer_result_type_from_database_callback : SQL_Expression -> SQL_Type_Reference) =
         _ = [infer_result_type_from_database_callback]
         mapping = self.get_type_mapping
         target_value_type = mapping.sql_type_to_value_type target_type

distribution/lib/Standard/Table/0.0.0-dev/src/Columns_To_Keep.enso

@@ -0,0 +1,35 @@
+from Standard.Base import Vector, Text
+from Standard.Base.Metadata import make_single_choice, Widget
+
+## Specifies which columns to keep in a union operation.
+type Columns_To_Keep
+    ## All columns are kept.
+
+       If a column is present only in some of the tables, it is padded with
+       `Nothing` for tables where it is missing.
+    In_Any
+
+    ## Only columns that are present in all tables are kept.
+
+       If there are columns that are only present in some of the tables,
+       a problem is reported.
+    In_All
+
+    ## Specific list of column names to keep.
+
+       If a table does not have a column that is specified in the list, it is
+       padded with `Nothing` and a problem is reported.
+    In_List (column_names : Vector Text)
+
+    ## PRIVATE
+       Same as `In_Any`, but it will warn about columns that are not present in
+       all tables.
+    In_Any_Warn_On_Missing
+
+    ## PRIVATE
+       The default widget for `Columns_To_Keep`.
+       It does not display the internal `In_Any_Warn_On_Missing` variant, since
+       that variant is only meant to be used as the default value.
+    default_widget -> Widget =
+        make_single_choice <|
+            ["In_Any", "In_All", "In_List"].map c-> [c, ".."+c]

distribution/lib/Standard/Table/0.0.0-dev/src/Errors.enso

@@ -494,26 +494,42 @@ type Column_Type_Mismatch
 
 type No_Common_Type
     ## PRIVATE
-       An error indicating that no common type could be found.
+       An error indicating that no common type could be found, and the operation
+       could not be performed.
 
        Arguments:
        - types: The types that were tried to be unified.
        - related_column_name: The name of the resulting column that was being
          unified, if applicable.
     Error (types : Vector Value_Type) (related_column_name : Nothing|Text)
 
+    ## PRIVATE
+       A warning indicating that no common type could be found, so the operation
+       had to fall back to converting all values to text.
+    Warning_Convert_To_Text  (types : Vector Value_Type) (related_column_name:Text)
+
     ## PRIVATE
 
        Create a human-readable version of the error.
     to_display_text : Text
     to_display_text self =
         types = self.types.map .to_display_text . join ", "
         prefix = "No common type was found for types: "+types
-        infix = case self.related_column_name of
-            column_name : Text -> " when unifying column ["+column_name+"]."
-            _ -> "."
-        suffix = " If you want to allow mixed types, please cast one of the columns to `Mixed` beforehand."
-        prefix + infix + suffix
+        location = case self.related_column_name of
+            column_name : Text -> " when unifying column ["+column_name+"]"
+            _ -> ""
+        suffix_type = case self of
+            No_Common_Type.Error _ _ -> "."
+            No_Common_Type.Warning_Convert_To_Text _ _ -> ", so the values were converted to text."
+        suffix_mixed = " If you want to have mixed types instead, please cast one of the columns to `Mixed` beforehand."
+        prefix + location + suffix_type + suffix_mixed
+
+    ## PRIVATE
+    to_text self -> Text =
+        ctor = case self of
+            No_Common_Type.Error _ _ -> "Error"
+            No_Common_Type.Warning_Convert_To_Text  _ _ -> "Warning_Convert_To_Text"
+        "No_Common_Type."+ctor+" "+self.types.to_text+" "+self.related_column_name.to_text
 
 type Unmatched_Columns
     ## PRIVATE
@@ -637,9 +653,11 @@ type Conversion_Failure
 
 type Loss_Of_Integer_Precision
     ## PRIVATE
-       Indicates that an automatic conversion of an integer column to a decimal
+       Indicates that an automatic conversion of an Integer column to a Float
        column is losing precision because some of the large integers cannot be
-       exactly represented by the `double` type.
+       exactly represented by the floating-point type.
+
+       Currently, this error is only reported in-memory.
     Warning (affected_rows_count : Integer) (example_value : Integer) (example_value_converted : Float)
 
     ## PRIVATE
@@ -834,3 +852,23 @@ type Nothing_Value_In_Filter_Condition
     to_display_text : Text
     to_display_text self =
         "Using `Nothing` as an argument to a `"+self.filter_condition.to_text+"` cannot match anything."
+
+## Indicates that different Date_Time (with or without timezone) or Date types
+   are mixed in the result, causing implicit coercions.
+
+   This is a warning, because using the `00:00` time and default time-zone may
+   not always be the expected choice, so the user should be aware of this.
+type Mixing_Date_Time_Types
+    ## PRIVATE
+    Date_To_Date_Time (related_column_name : Text | Nothing)
+
+    ## PRIVATE
+    Implicit_Time_Zone (related_column_name : Text | Nothing)
+
+    to_display_text self -> Text =
+        location = if self.related_column_name.is_nothing then "" else " (in column ["+self.related_column_name+"])"
+        case self of
+            Mixing_Date_Time_Types.Date_To_Date_Time _ ->
+                "Mixing Date and Date_Time values"+location+": the Date values have been automatically converted to Date_Time by adding a time of 00:00 in the default time-zone."
+            Mixing_Date_Time_Types.Implicit_Time_Zone _ ->
+                "Mixing Date_Time values with and without timezone"+location+". A default timezone has been assumed where it was missing."

distribution/lib/Standard/Table/0.0.0-dev/src/Excel/Excel_Workbook.enso

@@ -12,6 +12,7 @@ from Standard.Base.Data.Filter_Condition import sql_like_to_regex
 from Standard.Base.Metadata.Choice import Option
 from Standard.Base.Metadata.Widget import Multiple_Choice, Single_Choice
 
+import project.Columns_To_Keep.Columns_To_Keep
 import project.Excel.Excel_Range.Excel_Range
 import project.Headers.Headers
 import project.Internal.Excel_Reader
@@ -331,15 +332,15 @@ type Excel_Workbook
             tables = sheet_names.map on_problems=on_problems address-> self.read address headers on_problems=on_problems
             case return of
                 Return_As.Table_Of_Tables -> Table.new [["Sheet Name", sheet_names], ["Table", tables]]
-                Return_As.Merged_Table match ->
+                Return_As.Merged_Table columns_to_keep match ->
                     first_tbl = tables.find t-> t != Nothing
                     if first_tbl == Nothing then Error.throw (Illegal_Argument.Error "No valid sheets found.") else
                         unique = first_tbl.column_naming_helper.create_unique_name_strategy
                         tables.each tbl-> if tbl != Nothing then unique.mark_used tbl.column_names
                         new_column_name = unique.make_unique "Sheet Name"
 
                         with_names = tables.zip sheet_names tbl->name-> if tbl == Nothing then Nothing else tbl.set name new_column_name . reorder_columns [new_column_name]
-                        result = Table.from_union (with_names.filter Filter_Condition.Not_Nothing) match keep_unmatched_columns=True
+                        result = Table.from_union (with_names.filter Filter_Condition.Not_Nothing) columns_to_keep=columns_to_keep match_columns=match
 
                         problem_builder = Problem_Builder.new
                         problem_builder.report_unique_name_strategy unique
@@ -359,4 +360,4 @@ type Return_As
     Table_Of_Tables
 
     ## All sheets are merged into a single table. A union operation is performed.
-    Merged_Table match:Match_Columns=Match_Columns.By_Name
+    Merged_Table (columns_to_keep : Columns_To_Keep = Columns_To_Keep.In_Any) (match : Match_Columns = Match_Columns.By_Name)

distribution/lib/Standard/Table/0.0.0-dev/src/Internal/Table_Helpers.enso

@@ -10,6 +10,7 @@ import project.Expression.Expression
 import project.Internal.Column_Naming_Helper.Column_Naming_Helper
 import project.Internal.Problem_Builder.Problem_Builder
 import project.Internal.Value_Type_Helpers
+import project.Match_Columns.Column_Set
 import project.Position.Position
 import project.Set_Mode.Set_Mode
 import project.Sort_Column.Sort_Column
@@ -530,30 +531,38 @@ is_column obj =
 ## PRIVATE
    A helper method that resolves what should be the result type of a particular
    column set based on the union settings.
-unify_result_type_for_union column_set all_tables allow_type_widening problem_builder =
+unify_result_type_for_union (column_set : Column_Set) (all_tables : Vector) (problem_builder : Problem_Builder) -> Union_Result_Type =
     columns = column_set.resolve_columns all_tables
-    case allow_type_widening of
-        True ->
-            types = columns.filter Filter_Condition.Not_Nothing . map .value_type
-            common_type = Value_Type_Helpers.find_common_type types strict=True
-            if common_type.is_nothing then
-                problem_builder.report_other_warning (No_Common_Type.Error types related_column_name=column_set.name)
-            common_type
-        False ->
-            is_not_nothing c = case c of
-                Nothing -> False
-                _ -> True
-            first_column = columns.find is_not_nothing
-            first_type = first_column.value_type
-            if first_type == Value_Type.Mixed then Value_Type.Mixed else
-                first_wrong_column = columns.find if_missing=Nothing col->
-                    is_not_nothing col && col.value_type != first_type
-                case first_wrong_column of
-                    Nothing -> first_type
-                    _ ->
-                        got_type = first_wrong_column.value_type
-                        problem_builder.report_other_warning (Column_Type_Mismatch.Error column_set.name first_type got_type)
-                        Nothing
+        . filter Filter_Condition.Not_Nothing
+    types = columns.map .value_type
+
+    if types.is_empty then Union_Result_Type.No_Types_To_Unify else
+        ## First we check if we can find a generic common type.
+           This includes widening numeric column sizes, or converting Integer to Float.
+        common_type = Value_Type_Helpers.find_common_type types strict=True
+        if common_type.is_nothing.not then Union_Result_Type.Common_Type common_type else
+            ## Union has less strict requirements than other operations relying on `find_common_type`,
+               so if the common type was not found, we still check some fallbacks.
+            common_numeric_boolean = Value_Type_Helpers.find_common_numeric_boolean_type types
+            if common_numeric_boolean.is_nothing.not then Union_Result_Type.Common_Type common_numeric_boolean else
+                common_date_type = Value_Type_Helpers.find_common_date_types types column_set.name problem_builder
+                if common_date_type.is_nothing.not then Union_Result_Type.Common_Type common_date_type else
+                    # Lastly, we fall back to text, reporting a warning.
+                    problem_builder.report_other_warning (No_Common_Type.Warning_Convert_To_Text types column_set.name)
+                    Union_Result_Type.Fallback_To_Text
+
+## PRIVATE
+type Union_Result_Type
+    ## PRIVATE
+    Common_Type (value_type : Value_Type)
+
+    ## PRIVATE
+    Fallback_To_Text
+
+    ## PRIVATE
+       This case is returned if the requested column was missing from _all_ tables,
+       so there were no types to unify. An all-null column should be created.
+    No_Types_To_Unify
 
 ## PRIVATE
    Replace a set of columns in the table with a new set of columns. The old