feat(spansql): add support for STARTS_WITH function

- Adds support for the STARTS_WITH function.
- Adds an example for how further functions can be implemented.

Fixes #4661

spanner/spannertest/db_eval.go

@@ -28,6 +28,8 @@ import (
 
 	"cloud.google.com/go/civil"
 	"cloud.google.com/go/spanner/spansql"
+	"google.golang.org/grpc/codes"
+	"google.golang.org/grpc/status"
 )
 
 // evalContext represents the context for evaluating an expression.
@@ -72,7 +74,7 @@ func (ec evalContext) evalBoolExpr(be spansql.BoolExpr) (*bool, error) {
 	case spansql.BoolLiteral:
 		b := bool(be)
 		return &b, nil
-	case spansql.ID, spansql.Param, spansql.Paren, spansql.InOp: // InOp is a bit weird.
+	case spansql.ID, spansql.Param, spansql.Paren, spansql.Func, spansql.InOp: // InOp is a bit weird.
 		e, err := ec.evalExpr(be)
 		if err != nil {
 			return nil, err
@@ -363,6 +365,21 @@ func (ec evalContext) evalArithOp(e spansql.ArithOp) (interface{}, error) {
 	return nil, fmt.Errorf("TODO: evalArithOp(%s %v)", e.SQL(), e.Op)
 }
 
+func (ec evalContext) evalFunc(e spansql.Func) (interface{}, spansql.Type, error) {
+	if f, ok := functions[e.Name]; ok {
+		args := make([]interface{}, len(e.Args))
+		for i, arg := range e.Args {
+			val, err := ec.evalExpr(arg)
+			if err != nil {
+				return nil, spansql.Type{}, err
+			}
+			args[i] = val
+		}
+		return f.Eval(args)
+	}
+	return nil, spansql.Type{}, status.Errorf(codes.Unimplemented, "function %q is not implemented", e.Name)
+}
+
 // evalFloat64 evaluates an expression and returns its FLOAT64 value.
 // If the expression does not yield a FLOAT64 or INT64 it returns an error.
 func (ec evalContext) evalFloat64(e spansql.Expr) (float64, error) {
@@ -428,6 +445,12 @@ func (ec evalContext) evalExpr(e spansql.Expr) (interface{}, error) {
 		return bool(e), nil
 	case spansql.Paren:
 		return ec.evalExpr(e.Expr)
+	case spansql.Func:
+		v, _, err := ec.evalFunc(e)
+		if err != nil {
+			return nil, err
+		}
+		return v, nil
 	case spansql.Array:
 		var arr []interface{}
 		for _, elt := range e {
@@ -785,6 +808,12 @@ func (ec evalContext) colInfo(e spansql.Expr) (colInfo, error) {
 		return colInfo{Type: qp.Type}, nil
 	case spansql.Paren:
 		return ec.colInfo(e.Expr)
+	case spansql.Func:
+		_, t, err := ec.evalFunc(e)
+		if err != nil {
+			return colInfo{}, err
+		}
+		return colInfo{Type: t}, nil
 	case spansql.Array:
 		// Assume all element of an array literal have the same type.
 		if len(e) == 0 {

spanner/spannertest/funcs.go

@@ -19,12 +19,41 @@ package spannertest
 import (
 	"fmt"
 	"math"
+	"strings"
 
 	"cloud.google.com/go/spanner/spansql"
+	"google.golang.org/grpc/codes"
+	"google.golang.org/grpc/status"
 )
 
 // This file contains implementations of query functions.
 
+type function struct {
+	// Eval evaluates the result of the function using the given input.
+	Eval func(values []interface{}) (interface{}, spansql.Type, error)
+}
+
+var functions = map[string]function{
+	"STARTS_WITH": {
+		Eval: func(values []interface{}) (interface{}, spansql.Type, error) {
+			// TODO: Refine error messages to exactly match Spanner.
+			// Check input values first.
+			if len(values) != 2 {
+				return nil, spansql.Type{}, status.Error(codes.InvalidArgument, "No matching signature for function STARTS_WITH for the given argument types")
+			}
+			for _, v := range values {
+				// TODO: STARTS_WITH also supports BYTES as input parameters.
+				if _, ok := v.(string); !ok {
+					return nil, spansql.Type{}, status.Error(codes.InvalidArgument, "No matching signature for function STARTS_WITH for the given argument types")
+				}
+			}
+			s := values[0].(string)
+			prefix := values[1].(string)
+			return strings.HasPrefix(s, prefix), spansql.Type{Base: spansql.Bool}, nil
+		},
+	},
+}
+
 type aggregateFunc struct {
 	// Whether the function can take a * arg (only COUNT).
 	AcceptStar bool

spanner/spannertest/integration_test.go

@@ -748,9 +748,9 @@ func TestIntegration_ReadsAndQueries(t *testing.T) {
 		want   [][]interface{}
 	}{
 		{
-			`SELECT 17, "sweet", TRUE AND FALSE, NULL, B"hello"`,
+			`SELECT 17, "sweet", TRUE AND FALSE, NULL, B"hello", STARTS_WITH('Foo', 'B'), STARTS_WITH('Bar', 'B')`,
 			nil,
-			[][]interface{}{{int64(17), "sweet", false, nil, []byte("hello")}},
+			[][]interface{}{{int64(17), "sweet", false, nil, []byte("hello"), false, true}},
 		},
 		// Check handling of NULL values for the IS operator.
 		// There was a bug that returned errors for some of these cases.
@@ -824,6 +824,13 @@ func TestIntegration_ReadsAndQueries(t *testing.T) {
 				{"Sam"},
 			},
 		},
+		{
+			`SELECT Name FROM Staff WHERE STARTS_WITH(Name, 'Ja')`,
+			nil,
+			[][]interface{}{
+				{"Jack"},
+			},
+		},
 		{
 			`SELECT Name, Height FROM Staff WHERE Height BETWEEN @min AND @max ORDER BY Height DESC`,
 			map[string]interface{}{"min": 1.75, "max": 1.85},

spanner/spansql/parser_test.go

@@ -112,6 +112,15 @@ func TestParseQuery(t *testing.T) {
 				},
 			},
 		},
+		{`SELECT * FROM Foo WHERE STARTS_WITH(Bar, 'B')`,
+			Query{
+				Select: Select{
+					List:  []Expr{Star},
+					From:  []SelectFrom{SelectFromTable{Table: "Foo"}},
+					Where: Func{Name: "STARTS_WITH", Args: []Expr{ID("Bar"), StringLiteral("B")}},
+				},
+			},
+		},
 		{`SELECT SUM(PointsScored) AS total_points, FirstName, LastName AS surname FROM PlayerStats GROUP BY FirstName, LastName`,
 			Query{
 				Select: Select{
@@ -314,6 +323,9 @@ func TestParseExpr(t *testing.T) {
 		{`@needle IN UNNEST(@haystack)`, InOp{LHS: Param("needle"), RHS: []Expr{Param("haystack")}, Unnest: true}},
 		{`@needle NOT IN UNNEST(@haystack)`, InOp{LHS: Param("needle"), Neg: true, RHS: []Expr{Param("haystack")}, Unnest: true}},
 
+		// Functions
+		{`STARTS_WITH(Bar, 'B')`, Func{Name: "STARTS_WITH", Args: []Expr{ID("Bar"), StringLiteral("B")}}},
+
 		// String literal:
 		// Accept double quote and single quote.
 		{`"hello"`, StringLiteral("hello")},