Bug fix and tests for https://github.com/dolthub/dolt/issues/6391

go/libraries/doltcore/sqle/dtables/diff_table.go

@@ -98,6 +98,13 @@ func NewDiffTable(ctx *sql.Context, tblName string, ddb *doltdb.DoltDB, root *do
 		return nil, err
 	}
 
+	// Process sch to widen all the fields so that any previous values (in the same type family) can be
+	// shown in the diff table, even if the current schema's type has been changed to a smaller type.
+	sch, err = widenSchemaColumns(sch)
+	if err != nil {
+		return nil, err
+	}
+
 	diffTableSchema, j, err := GetDiffTableSchemaAndJoiner(ddb.Format(), sch, sch)
 	if err != nil {
 		return nil, err
@@ -398,6 +405,21 @@ func (dt *DiffTable) reverseIterForChild(ctx *sql.Context, parent hash.Hash) (*d
 	}
 }
 
+// widenSchemaColumns takes a schema, |sch|, and returns a new schema with all the non-PK columns
+// promoted to their widest type in the same family (e.g. tinyint -> bigint, varchar(10) -> TEXT).
+// This function is used so that when a table's schema has changed throughout its history, we can
+// still show any previous values in the diff table, even if those previous values are larger than
+// the current schema's type.
+func widenSchemaColumns(sch schema.Schema) (schema.Schema, error) {
+	widenedCols := make([]schema.Column, 0, sch.GetAllCols().Size())
+	for _, col := range sch.GetAllCols().GetColumns() {
+		col.TypeInfo = col.TypeInfo.Promote()
+		widenedCols = append(widenedCols, col)
+	}
+	return schema.NewSchema(schema.NewColCollection(widenedCols...),
+		[]int{}, sch.GetCollation(), sch.Indexes(), sch.Checks())
+}
+
 func tableInfoForCommit(ctx context.Context, table string, cm *doltdb.Commit, hs hash.Hash) (TblInfoAtCommit, error) {
 	r, err := cm.GetRootValue(ctx)
 	if err != nil {
@@ -706,17 +728,6 @@ type DiffPartitions struct {
 	fromSch         schema.Schema
 }
 
-func NewDiffPartitions(tblName string, cmItr doltdb.CommitItr, cmHashToTblInfo map[hash.Hash]TblInfoAtCommit, selectFunc partitionSelectFunc, toSch, fromSch schema.Schema) *DiffPartitions {
-	return &DiffPartitions{
-		tblName:         tblName,
-		cmItr:           cmItr,
-		cmHashToTblInfo: cmHashToTblInfo,
-		selectFunc:      selectFunc,
-		toSch:           toSch,
-		fromSch:         fromSch,
-	}
-}
-
 // processCommit is called in a commit iteration loop. Adds partitions when it finds a commit and its parent that have
 // different values for the hash of the table being looked at.
 func (dps *DiffPartitions) processCommit(ctx *sql.Context, cmHash hash.Hash, cm *doltdb.Commit, root *doltdb.RootValue, tbl *doltdb.Table) (*DiffPartition, error) {

go/libraries/doltcore/sqle/enginetest/dolt_queries_diff.go

@@ -402,6 +402,29 @@ var DiffSystemTableScriptTests = []queries.ScriptTest{
 			},
 		},
 	},
+	{
+		// https://github.com/dolthub/dolt/issues/6391
+		Name: "columns modified to narrower types",
+		SetUpScript: []string{
+			"create table t (pk int primary key, col1 varchar(20), col2 int);",
+			"call dolt_commit('-Am', 'new table t');",
+			"insert into t values (1, '123456789012345', 420);",
+			"call dolt_commit('-am', 'inserting data');",
+			"update t set col1='1234567890', col2=13;",
+			"alter table t modify column col1 varchar(10);",
+			"alter table t modify column col2 tinyint;",
+			"call dolt_commit('-am', 'narrowing types');",
+		},
+		Assertions: []queries.ScriptTestAssertion{
+			{
+				Query: "select to_pk, to_col1, to_col2, to_commit, from_pk, from_col1, from_col2, from_commit, diff_type from dolt_diff_t order by from_commit_date ASC;",
+				Expected: []sql.Row{
+					{1, "123456789012345", 420, doltCommit, nil, nil, nil, doltCommit, "added"},
+					{1, "1234567890", 13, doltCommit, 1, "123456789012345", 420, doltCommit, "modified"},
+				},
+			},
+		},
+	},
 	{
 		Name: "multiple column renames",
 		SetUpScript: []string{

integration-tests/bats/system-tables.bats

@@ -436,6 +436,22 @@ SQL
     [[ "$output" =~ "$EXPECTED" ]] || false
 }
 
+@test "system-tables: query dolt_diff_ system table when column types have been narrowed" {
+    dolt sql -q "create table t (pk int primary key, col1 varchar(20), col2 int);"
+    dolt commit -Am "new table t"
+    dolt sql -q "insert into t values (1, '123456789012345', 420);"
+    dolt commit -am "inserting a row"
+    dolt sql -q "update t set col1='1234567890', col2=13;"
+    dolt sql -q "alter table t modify column col1 varchar(10);"
+    dolt sql -q "alter table t modify column col2 tinyint;"
+    dolt commit -am "narrowing types"
+
+    run dolt sql -r csv -q "select to_pk, to_col1, to_col2, from_pk, from_col1, from_col2, diff_type from dolt_diff_t order by from_commit_date ASC;"
+    [ $status -eq 0 ]
+    [[ $output =~ "1,123456789012345,420,,,,added" ]] || false
+    [[ $output =~ "1,1234567890,13,1,123456789012345,420,modified" ]] || false
+}
+
 @test "system-tables: query dolt_history_ system table" {
     dolt sql -q "create table test (pk int, c1 int, primary key(pk))"
     dolt add test