Fix full_scan_algorithm comments.

Adjust comments to show that split query no longer preserves WHERE
clause. Add ORDER BY clause to comments.

Adjust LIMIT to be numRowsPerQueryPart-1, 1 for correctness.

go/vt/tabletserver/splitquery/full_scan_algorithm.go

@@ -15,15 +15,15 @@ import (
 // It iteratively executes the following query over the replica’s database (recall that MySQL
 // performs tuple comparisons lexicographically):
 //    SELECT <split_columns> FROM <table>
-//                           WHERE (<where>) AND (:prev_boundary <= (<split_columns>))
+//                           WHERE :prev_boundary <= (<split_columns>)
 //                           ORDER BY <split_columns>
 //                           LIMIT <num_rows_per_query_part>, 1
-// where <split_columns> denotes the ordered list of split columns, and <table> and <where> are the
-// values of the FROM and WHERE clauses of the input query, respectefully.
+// where <split_columns> denotes the ordered list of split columns, and <table> is the
+// value of the FROM and WHERE clauses of the input query, respectfully.
 // The 'prev_boundary' bind variable holds a tuple consisting of split column values.
 // It is updated after each iteration with the result of the query. In the query executed in the
-// first iteration (the initial query) the term 'AND (:prev_boundary <= (<split_columns>))' is
-// ommitted.
+// first iteration (the initial query) the term ':prev_boundary <= (<split_columns>)' is
+// omitted.
 // The algorithm stops when the query returns no results. The result of this algorithm is the list
 // consisting of the result of each query in order.
 //
@@ -108,8 +108,8 @@ func (a *FullScanAlgorithm) populatePrevTupleInBindVariables(
 //    "SELECT <select exprs> FROM <table> WHERE <where>",
 // the Sql field of the result will be:
 // "SELECT sc_1,sc_2,...,sc_n FROM <table>
-//                            WHERE <where>
-//                            LIMIT splitParams.numRowsPerQueryPart, 1",
+//                            ORDER BY <split_columns>
+//                            LIMIT splitParams.numRowsPerQueryPart-1, 1",
 // The BindVariables field of the result will contain a deep-copy of splitParams.BindVariables.
 func buildInitialQuery(splitParams *SplitParams) *querytypes.BoundQuery {
 	resultSelectAST := buildInitialQueryAST(splitParams)
@@ -128,7 +128,7 @@ func buildInitialQueryAST(splitParams *SplitParams) *sqlparser.Select {
 	resultSelectAST := *splitParams.selectAST
 	resultSelectAST.Where = nil
 	resultSelectAST.SelectExprs = convertColumnsToSelectExprs(splitParams.splitColumns)
-	resultSelectAST.Limit = buildLimitClause(splitParams.numRowsPerQueryPart, 1)
+	resultSelectAST.Limit = buildLimitClause(splitParams.numRowsPerQueryPart-1, 1)
 	resultSelectAST.OrderBy = buildOrderByClause(splitParams.splitColumns)
 	return &resultSelectAST
 }
@@ -139,7 +139,8 @@ func buildInitialQueryAST(splitParams *SplitParams) *sqlparser.Select {
 //    "SELECT <select exprs> FROM <table> WHERE <where>",
 // the Sql field of the result will be:
 // "SELECT sc_1,sc_2,...,sc_n FROM <table>
-//                            WHERE (<where>) AND (:prev_sc_1,...,:prev_sc_n) <= (sc_1,...,sc_n)
+//                            WHERE :prev_sc_1,...,:prev_sc_n) <= (sc_1,...,sc_n)
+//                            ORDER BY <split_columns>
 //                            LIMIT splitParams.numRowsPerQueryPart, 1",
 // where sc_1,...,sc_n are the split columns,
 // and :prev_sc_1,...,:_prev_sc_n are the bind variable names for the previous tuple.

go/vt/tabletserver/splitquery/full_scan_algorithm_test.go

@@ -31,7 +31,7 @@ func TestMultipleBoundaries(t *testing.T) {
 	expectedCall1 := mockSQLExecuter.EXPECT().SQLExecute(
 		"select id, user_id from test_table"+
 			" order by id asc, user_id asc"+
-			" limit 1000, 1",
+			" limit 999, 1",
 		map[string]interface{}{})
 	expectedCall1.Return(
 		&sqltypes.Result{
@@ -45,7 +45,7 @@ func TestMultipleBoundaries(t *testing.T) {
 			" :_splitquery_prev_id < id or"+
 			" (:_splitquery_prev_id = id and :_splitquery_prev_user_id <= user_id)"+
 			" order by id asc, user_id asc"+
-			" limit 1000, 1",
+			" limit 999, 1",
 		map[string]interface{}{
 			"_splitquery_prev_id":      int64(1),
 			"_splitquery_prev_user_id": int64(1),
@@ -63,7 +63,7 @@ func TestMultipleBoundaries(t *testing.T) {
 			" :_splitquery_prev_id < id or"+
 			" (:_splitquery_prev_id = id and :_splitquery_prev_user_id <= user_id)"+
 			" order by id asc, user_id asc"+
-			" limit 1000, 1",
+			" limit 999, 1",
 		map[string]interface{}{
 			"_splitquery_prev_id":      int64(2),
 			"_splitquery_prev_user_id": int64(10),
@@ -109,7 +109,7 @@ func TestSmallNumberOfRows(t *testing.T) {
 	expectedCall1 := mockSQLExecuter.EXPECT().SQLExecute(
 		"select id, user_id from test_table"+
 			" order by id asc, user_id asc"+
-			" limit 1000, 1",
+			" limit 999, 1",
 		map[string]interface{}{})
 	expectedCall1.Return(
 		&sqltypes.Result{Rows: [][]sqltypes.Value{}}, nil)
@@ -148,7 +148,7 @@ func TestSQLExecuterReturnsError(t *testing.T) {
 	expectedCall1 := mockSQLExecuter.EXPECT().SQLExecute(
 		"select id, user_id from test_table"+
 			" order by id asc, user_id asc"+
-			" limit 1000, 1",
+			" limit 999, 1",
 		map[string]interface{}{})
 	expectedCall1.Return(
 		&sqltypes.Result{
@@ -162,7 +162,7 @@ func TestSQLExecuterReturnsError(t *testing.T) {
 			" :_splitquery_prev_id < id or"+
 			" (:_splitquery_prev_id = id and :_splitquery_prev_user_id <= user_id)"+
 			" order by id asc, user_id asc"+
-			" limit 1000, 1",
+			" limit 999, 1",
 		map[string]interface{}{
 			"_splitquery_prev_id":      int64(1),
 			"_splitquery_prev_user_id": int64(1),