dropped columns are not 'shared' and no data copy attempted for such columns

go/base/context.go

@@ -181,6 +181,7 @@ type MigrationContext struct {
 	UniqueKey                        *sql.UniqueKey
 	SharedColumns                    *sql.ColumnList
 	ColumnRenameMap                  map[string]string
+	DroppedColumnsMap                map[string]bool
 	MappedSharedColumns              *sql.ColumnList
 	MigrationRangeMinValues          *sql.ColumnValues
 	MigrationRangeMaxValues          *sql.ColumnValues

go/logic/inspect.go

@@ -662,7 +662,14 @@ func (this *Inspector) getSharedColumns(originalColumns, ghostColumns *sql.Colum
 	}
 	sharedColumnNames := []string{}
 	for _, originalColumn := range originalColumns.Names() {
+		isSharedColumn := false
 		if columnsInGhost[originalColumn] || columnsInGhost[columnRenameMap[originalColumn]] {
+			isSharedColumn = true
+		}
+		if this.migrationContext.DroppedColumnsMap[originalColumn] {
+			isSharedColumn = false
+		}
+		if isSharedColumn {
 			sharedColumnNames = append(sharedColumnNames, originalColumn)
 		}
 	}

go/logic/migrator.go

@@ -248,6 +248,7 @@ func (this *Migrator) validateStatement() (err error) {
 		}
 		log.Infof("Alter statement has column(s) renamed. gh-ost finds the following renames: %v; --approve-renamed-columns is given and so migration proceeds.", this.parser.GetNonTrivialRenames())
 	}
+	this.migrationContext.DroppedColumnsMap = this.parser.DroppedColumnsMap()
 	return nil
 }
 

go/sql/parser.go

@@ -111,3 +111,7 @@ func (this *Parser) GetNonTrivialRenames() map[string]string {
 func (this *Parser) HasNonTrivialRenames() bool {
 	return len(this.GetNonTrivialRenames()) > 0
 }
+
+func (this *Parser) DroppedColumnsMap() map[string]bool {
+	return this.droppedColumns
+}