db.go

/*
Copyright 2019 Google LLC

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

package spannertest

// This file contains the implementation of the Spanner fake itself,
// namely the part behind the RPC interface.

// TODO: missing transactionality in a serious way!

import (
	"bytes"
	"encoding/base64"
	"fmt"
	"sort"
	"strconv"
	"strings"
	"sync"
	"time"

	"google.golang.org/grpc/codes"
	"google.golang.org/grpc/status"

	structpb "github.com/golang/protobuf/ptypes/struct"

	"cloud.google.com/go/civil"
	"cloud.google.com/go/spanner/spansql"
)

type database struct {
	mu      sync.Mutex
	lastTS  time.Time // last commit timestamp
	tables  map[spansql.ID]*table
	indexes map[spansql.ID]struct{} // only record their existence
	views   map[spansql.ID]struct{} // only record their existence

	rwMu sync.Mutex // held by read-write transactions
}

type table struct {
	mu sync.Mutex

	// Information about the table columns.
	// They are reordered on table creation so the primary key columns come first.
	cols      []colInfo
	colIndex  map[spansql.ID]int         // col name to index
	origIndex map[spansql.ID]int         // original index of each column upon construction
	pkCols    int                        // number of primary key columns (may be 0)
	pkDesc    []bool                     // whether each primary key column is in descending order
	rdw       *spansql.RowDeletionPolicy // RowDeletionPolicy of this table (may be nil)

	// Rows are stored in primary key order.
	rows []row
}

// colInfo represents information about a column in a table or result set.
type colInfo struct {
	Name      spansql.ID
	Type      spansql.Type
	Generated spansql.Expr
	NotNull   bool            // only set for table columns
	AggIndex  int             // Index+1 of SELECT list for which this is an aggregate value.
	Alias     spansql.PathExp // an alternate name for this column (result sets only)
}

// commitTimestampSentinel is a sentinel value for TIMESTAMP fields with allow_commit_timestamp=true.
// It is accepted, but never stored.
var commitTimestampSentinel = &struct{}{}

// transaction records information about a running transaction.
// This is not safe for concurrent use.
type transaction struct {
	// readOnly is whether this transaction was constructed
	// for read-only use, and should yield errors if used
	// to perform a mutation.
	readOnly bool

	d               *database
	commitTimestamp time.Time // not set if readOnly
	unlock          func()    // may be nil
}

func (d *database) NewReadOnlyTransaction() *transaction {
	return &transaction{
		readOnly: true,
	}
}

func (d *database) NewTransaction() *transaction {
	return &transaction{
		d: d,
	}
}

// Start starts the transaction and commits to a specific commit timestamp.
// This also locks out any other read-write transaction on this database
// until Commit/Rollback are called.
func (tx *transaction) Start() {
	// Commit timestamps are only guaranteed to be unique
	// when transactions write to overlapping sets of fields.
	// This simulated database exceeds that guarantee.

	// Grab rwMu for the duration of this transaction.
	// Take it before d.mu so we don't hold that lock
	// while waiting for d.rwMu, which is held for longer.
	tx.d.rwMu.Lock()

	tx.d.mu.Lock()
	const tsRes = 1 * time.Microsecond
	now := time.Now().UTC().Truncate(tsRes)
	if !now.After(tx.d.lastTS) {
		now = tx.d.lastTS.Add(tsRes)
	}
	tx.d.lastTS = now
	tx.d.mu.Unlock()

	tx.commitTimestamp = now
	tx.unlock = tx.d.rwMu.Unlock
}

func (tx *transaction) checkMutable() error {
	if tx.readOnly {
		// TODO: is this the right status?
		return status.Errorf(codes.InvalidArgument, "transaction is read-only")
	}
	return nil
}

func (tx *transaction) Commit() (time.Time, error) {
	if tx.unlock != nil {
		tx.unlock()
	}
	return tx.commitTimestamp, nil
}

func (tx *transaction) Rollback() {
	if tx.unlock != nil {
		tx.unlock()
	}
	// TODO: actually rollback
}

/*
row represents a list of data elements.

The mapping between Spanner types and Go types internal to this package are:
	BOOL		bool
	INT64		int64
	FLOAT64		float64
	STRING		string
	BYTES		[]byte
	DATE		civil.Date
	TIMESTAMP	time.Time (location set to UTC)
	ARRAY<T>	[]interface{}
	STRUCT		TODO
*/
type row []interface{}

func (r row) copyDataElem(index int) interface{} {
	v := r[index]
	if is, ok := v.([]interface{}); ok {
		// Deep-copy array values.
		v = append([]interface{}(nil), is...)
	}
	return v
}

// copyData returns a copy of the row.
func (r row) copyAllData() row {
	dst := make(row, 0, len(r))
	for i := range r {
		dst = append(dst, r.copyDataElem(i))
	}
	return dst
}

// copyData returns a copy of a subset of a row.
func (r row) copyData(indexes []int) row {
	if len(indexes) == 0 {
		return nil
	}
	dst := make(row, 0, len(indexes))
	for _, i := range indexes {
		dst = append(dst, r.copyDataElem(i))
	}
	return dst
}

func (d *database) LastCommitTimestamp() time.Time {
	d.mu.Lock()
	defer d.mu.Unlock()
	return d.lastTS
}

func (d *database) GetDDL() []spansql.DDLStmt {
	// This lacks fidelity, but captures the details we support.
	d.mu.Lock()
	defer d.mu.Unlock()

	var stmts []spansql.DDLStmt

	for name, t := range d.tables {
		ct := &spansql.CreateTable{
			Name: name,
		}

		t.mu.Lock()
		for i, col := range t.cols {
			ct.Columns = append(ct.Columns, spansql.ColumnDef{
				Name:    col.Name,
				Type:    col.Type,
				NotNull: col.NotNull,
				// TODO: AllowCommitTimestamp
			})
			if i < t.pkCols {
				ct.PrimaryKey = append(ct.PrimaryKey, spansql.KeyPart{
					Column: col.Name,
					Desc:   t.pkDesc[i],
				})
			}
		}
		t.mu.Unlock()

		stmts = append(stmts, ct)
	}

	return stmts
}

func (d *database) ApplyDDL(stmt spansql.DDLStmt) *status.Status {
	d.mu.Lock()
	defer d.mu.Unlock()

	// Lazy init.
	if d.tables == nil {
		d.tables = make(map[spansql.ID]*table)
	}
	if d.indexes == nil {
		d.indexes = make(map[spansql.ID]struct{})
	}
	if d.views == nil {
		d.views = make(map[spansql.ID]struct{})
	}

	switch stmt := stmt.(type) {
	default:
		return status.Newf(codes.Unimplemented, "unhandled DDL statement type %T", stmt)
	case *spansql.CreateTable:
		if _, ok := d.tables[stmt.Name]; ok {
			return status.Newf(codes.AlreadyExists, "table %s already exists", stmt.Name)
		}
		if len(stmt.PrimaryKey) == 0 {
			return status.Newf(codes.InvalidArgument, "table %s has no primary key", stmt.Name)
		}

		// TODO: check stmt.Interleave details.

		// Record original column ordering.
		orig := make(map[spansql.ID]int)
		for i, col := range stmt.Columns {
			orig[col.Name] = i
		}

		// Move primary keys first, preserving their order.
		pk := make(map[spansql.ID]int)
		var pkDesc []bool
		for i, kp := range stmt.PrimaryKey {
			pk[kp.Column] = -1000 + i
			pkDesc = append(pkDesc, kp.Desc)
		}
		sort.SliceStable(stmt.Columns, func(i, j int) bool {
			a, b := pk[stmt.Columns[i].Name], pk[stmt.Columns[j].Name]
			return a < b
		})

		t := &table{
			colIndex:  make(map[spansql.ID]int),
			origIndex: orig,
			pkCols:    len(pk),
			pkDesc:    pkDesc,
		}
		for _, cd := range stmt.Columns {
			if st := t.addColumn(cd, true); st.Code() != codes.OK {
				return st
			}
		}
		for col := range pk {
			if _, ok := t.colIndex[col]; !ok {
				return status.Newf(codes.InvalidArgument, "primary key column %q not in table", col)
			}
		}
		t.rdw = stmt.RowDeletionPolicy
		d.tables[stmt.Name] = t
		return nil
	case *spansql.CreateIndex:
		if _, ok := d.indexes[stmt.Name]; ok {
			return status.Newf(codes.AlreadyExists, "index %s already exists", stmt.Name)
		}
		d.indexes[stmt.Name] = struct{}{}
		return nil
	case *spansql.CreateView:
		if !stmt.OrReplace {
			if _, ok := d.views[stmt.Name]; ok {
				return status.Newf(codes.AlreadyExists, "view %s already exists", stmt.Name)
			}
		}
		d.views[stmt.Name] = struct{}{}
		return nil
	case *spansql.DropTable:
		if _, ok := d.tables[stmt.Name]; !ok {
			return status.Newf(codes.NotFound, "no table named %s", stmt.Name)
		}
		// TODO: check for indexes on this table.
		delete(d.tables, stmt.Name)
		return nil
	case *spansql.DropIndex:
		if _, ok := d.indexes[stmt.Name]; !ok {
			return status.Newf(codes.NotFound, "no index named %s", stmt.Name)
		}
		delete(d.indexes, stmt.Name)
		return nil
	case *spansql.DropView:
		if _, ok := d.views[stmt.Name]; !ok {
			return status.Newf(codes.NotFound, "no view named %s", stmt.Name)
		}
		delete(d.views, stmt.Name)
		return nil
	case *spansql.AlterTable:
		t, ok := d.tables[stmt.Name]
		if !ok {
			return status.Newf(codes.NotFound, "no table named %s", stmt.Name)
		}
		switch alt := stmt.Alteration.(type) {
		default:
			return status.Newf(codes.Unimplemented, "unhandled DDL table alteration type %T", alt)
		case spansql.AddColumn:
			if st := t.addColumn(alt.Def, false); st.Code() != codes.OK {
				return st
			}
			return nil
		case spansql.DropColumn:
			if st := t.dropColumn(alt.Name); st.Code() != codes.OK {
				return st
			}
			return nil
		case spansql.AlterColumn:
			if st := t.alterColumn(alt); st.Code() != codes.OK {
				return st
			}
			return nil
		case spansql.AddRowDeletionPolicy:
			if st := t.addRowDeletionPolicy(alt); st.Code() != codes.OK {
				return st
			}
			return nil
		case spansql.ReplaceRowDeletionPolicy:
			if st := t.replaceRowDeletionPolicy(alt); st.Code() != codes.OK {
				return st
			}
			return nil
		case spansql.DropRowDeletionPolicy:
			if st := t.dropRowDeletionPolicy(alt); st.Code() != codes.OK {
				return st
			}
			return nil
		}
	}

}

func (d *database) table(tbl spansql.ID) (*table, error) {
	d.mu.Lock()
	defer d.mu.Unlock()

	t, ok := d.tables[tbl]
	if !ok {
		return nil, status.Errorf(codes.NotFound, "no table named %s", tbl)
	}
	return t, nil
}

// writeValues executes a write option (Insert, Update, etc.).
func (d *database) writeValues(tx *transaction, tbl spansql.ID, cols []spansql.ID, values []*structpb.ListValue, f func(t *table, colIndexes []int, r row) error) error {
	if err := tx.checkMutable(); err != nil {
		return err
	}

	t, err := d.table(tbl)
	if err != nil {
		return err
	}

	t.mu.Lock()
	defer t.mu.Unlock()

	colIndexes, err := t.colIndexes(cols)
	if err != nil {
		return err
	}
	revIndex := make(map[int]int) // table index to col index
	for j, i := range colIndexes {
		revIndex[i] = j
	}

	for pki := 0; pki < t.pkCols; pki++ {
		_, ok := revIndex[pki]
		if !ok {
			return status.Errorf(codes.InvalidArgument, "primary key column %s not included in write", t.cols[pki].Name)
		}
	}

	for _, vs := range values {
		if len(vs.Values) != len(colIndexes) {
			return status.Errorf(codes.InvalidArgument, "row of %d values can't be written to %d columns", len(vs.Values), len(colIndexes))
		}

		r := make(row, len(t.cols))
		for j, v := range vs.Values {
			i := colIndexes[j]

			if t.cols[i].Generated != nil {
				return status.Error(codes.InvalidArgument, "values can't be written to a generated column")
			}
			x, err := valForType(v, t.cols[i].Type)
			if err != nil {
				return err
			}
			if x == commitTimestampSentinel {
				x = tx.commitTimestamp
			}
			if x == nil && t.cols[i].NotNull {
				return status.Errorf(codes.FailedPrecondition, "%s must not be NULL in table %s", t.cols[i].Name, tbl)
			}

			r[i] = x
		}
		// TODO: enforce that provided timestamp for commit_timestamp=true columns
		// are not ahead of the transaction's commit timestamp.

		if err := f(t, colIndexes, r); err != nil {
			return err
		}

		// Get row again after potential update merge to ensure we compute
		// generated columns with fresh data.
		pk := r[:t.pkCols]
		rowNum, found := t.rowForPK(pk)
		// This should never fail as the row was just inserted.
		if !found {
			return status.Error(codes.Internal, "row failed to be inserted")
		}
		row := t.rows[rowNum]
		ec := evalContext{
			cols: t.cols,
			row:  row,
		}

		// TODO: We would need to do a topological sort on dependencies
		// (i.e. what other columns the expression references) to ensure we
		// can handle generated columns which reference other generated columns
		for i, col := range t.cols {
			if col.Generated != nil {
				res, err := ec.evalExpr(col.Generated)
				if err != nil {
					return err
				}
				row[i] = res
			}
		}
	}

	return nil
}

func (d *database) Insert(tx *transaction, tbl spansql.ID, cols []spansql.ID, values []*structpb.ListValue) error {
	return d.writeValues(tx, tbl, cols, values, func(t *table, colIndexes []int, r row) error {
		pk := r[:t.pkCols]
		rowNum, found := t.rowForPK(pk)
		if found {
			return status.Errorf(codes.AlreadyExists, "row already in table")
		}
		t.insertRow(rowNum, r)
		return nil
	})
}

func (d *database) Update(tx *transaction, tbl spansql.ID, cols []spansql.ID, values []*structpb.ListValue) error {
	return d.writeValues(tx, tbl, cols, values, func(t *table, colIndexes []int, r row) error {
		if t.pkCols == 0 {
			return status.Errorf(codes.InvalidArgument, "cannot update table %s with no columns in primary key", tbl)
		}
		pk := r[:t.pkCols]
		rowNum, found := t.rowForPK(pk)
		if !found {
			// TODO: is this the right way to return `NOT_FOUND`?
			return status.Errorf(codes.NotFound, "row not in table")
		}

		for _, i := range colIndexes {
			t.rows[rowNum][i] = r[i]
		}
		return nil
	})
}

func (d *database) InsertOrUpdate(tx *transaction, tbl spansql.ID, cols []spansql.ID, values []*structpb.ListValue) error {
	return d.writeValues(tx, tbl, cols, values, func(t *table, colIndexes []int, r row) error {
		pk := r[:t.pkCols]
		rowNum, found := t.rowForPK(pk)
		if !found {
			// New row; do an insert.
			t.insertRow(rowNum, r)
		} else {
			// Existing row; do an update.
			for _, i := range colIndexes {
				t.rows[rowNum][i] = r[i]
			}
		}
		return nil
	})
}

// TODO: Replace

func (d *database) Delete(tx *transaction, table spansql.ID, keys []*structpb.ListValue, keyRanges keyRangeList, all bool) error {
	if err := tx.checkMutable(); err != nil {
		return err
	}

	t, err := d.table(table)
	if err != nil {
		return err
	}

	t.mu.Lock()
	defer t.mu.Unlock()

	if all {
		t.rows = nil
		return nil
	}

	for _, key := range keys {
		pk, err := t.primaryKey(key.Values)
		if err != nil {
			return err
		}
		// Not an error if the key does not exist.
		rowNum, found := t.rowForPK(pk)
		if found {
			copy(t.rows[rowNum:], t.rows[rowNum+1:])
			t.rows = t.rows[:len(t.rows)-1]
		}
	}

	for _, r := range keyRanges {
		r.startKey, err = t.primaryKeyPrefix(r.start.Values)
		if err != nil {
			return err
		}
		r.endKey, err = t.primaryKeyPrefix(r.end.Values)
		if err != nil {
			return err
		}
		startRow, endRow := t.findRange(r)
		if n := endRow - startRow; n > 0 {
			copy(t.rows[startRow:], t.rows[endRow:])
			t.rows = t.rows[:len(t.rows)-n]
		}
	}

	return nil
}

// readTable executes a read option (Read, ReadAll).
func (d *database) readTable(table spansql.ID, cols []spansql.ID, f func(*table, *rawIter, []int) error) (*rawIter, error) {
	t, err := d.table(table)
	if err != nil {
		return nil, err
	}

	t.mu.Lock()
	defer t.mu.Unlock()

	colIndexes, err := t.colIndexes(cols)
	if err != nil {
		return nil, err
	}

	ri := &rawIter{}
	for _, i := range colIndexes {
		ri.cols = append(ri.cols, t.cols[i])
	}
	return ri, f(t, ri, colIndexes)
}

func (d *database) Read(tbl spansql.ID, cols []spansql.ID, keys []*structpb.ListValue, keyRanges keyRangeList, limit int64) (rowIter, error) {
	// The real Cloud Spanner returns an error if the key set is empty by definition.
	// That doesn't seem to be well-defined, but it is a common error to attempt a read with no keys,
	// so catch that here and return a representative error.
	if len(keys) == 0 && len(keyRanges) == 0 {
		return nil, status.Error(codes.Unimplemented, "Cloud Spanner does not support reading no keys")
	}

	return d.readTable(tbl, cols, func(t *table, ri *rawIter, colIndexes []int) error {
		// "If the same key is specified multiple times in the set (for
		// example if two ranges, two keys, or a key and a range
		// overlap), Cloud Spanner behaves as if the key were only
		// specified once."
		done := make(map[int]bool) // row numbers we've included in ri.

		// Specific keys.
		for _, key := range keys {
			pk, err := t.primaryKey(key.Values)
			if err != nil {
				return err
			}
			// Not an error if the key does not exist.
			rowNum, found := t.rowForPK(pk)
			if !found {
				continue
			}
			if done[rowNum] {
				continue
			}
			done[rowNum] = true
			ri.add(t.rows[rowNum], colIndexes)
			if limit > 0 && len(ri.rows) >= int(limit) {
				return nil
			}
		}

		// Key ranges.
		for _, r := range keyRanges {
			var err error
			r.startKey, err = t.primaryKeyPrefix(r.start.Values)
			if err != nil {
				return err
			}
			r.endKey, err = t.primaryKeyPrefix(r.end.Values)
			if err != nil {
				return err
			}
			startRow, endRow := t.findRange(r)
			for rowNum := startRow; rowNum < endRow; rowNum++ {
				if done[rowNum] {
					continue
				}
				done[rowNum] = true
				ri.add(t.rows[rowNum], colIndexes)
				if limit > 0 && len(ri.rows) >= int(limit) {
					return nil
				}
			}
		}

		return nil
	})
}

func (d *database) ReadAll(tbl spansql.ID, cols []spansql.ID, limit int64) (*rawIter, error) {
	return d.readTable(tbl, cols, func(t *table, ri *rawIter, colIndexes []int) error {
		for _, r := range t.rows {
			ri.add(r, colIndexes)
			if limit > 0 && len(ri.rows) >= int(limit) {
				break
			}
		}
		return nil
	})
}

func (t *table) addColumn(cd spansql.ColumnDef, newTable bool) *status.Status {
	if !newTable && cd.NotNull {
		return status.Newf(codes.InvalidArgument, "new non-key columns cannot be NOT NULL")
	}

	if _, ok := t.colIndex[cd.Name]; ok {
		return status.Newf(codes.AlreadyExists, "column %s already exists", cd.Name)
	}

	t.mu.Lock()
	defer t.mu.Unlock()

	if len(t.rows) > 0 {
		if cd.NotNull {
			// TODO: what happens in this case?
			return status.Newf(codes.Unimplemented, "can't add NOT NULL columns to non-empty tables yet")
		}
		for i := range t.rows {
			if cd.Generated != nil {
				ec := evalContext{
					cols: t.cols,
					row:  t.rows[i],
				}
				val, err := ec.evalExpr(cd.Generated)
				if err != nil {
					return status.Newf(codes.InvalidArgument, "could not backfill values for generated column: %v", err)
				}
				t.rows[i] = append(t.rows[i], val)
			} else {
				t.rows[i] = append(t.rows[i], nil)
			}
		}
	}

	t.cols = append(t.cols, colInfo{
		Name:    cd.Name,
		Type:    cd.Type,
		NotNull: cd.NotNull,
		// TODO: We should figure out what columns the Generator expression
		// relies on and check it is valid at this time currently it will
		// fail when writing data instead as it is the first time we
		// evaluate the expression.
		Generated: cd.Generated,
	})
	t.colIndex[cd.Name] = len(t.cols) - 1
	if !newTable {
		t.origIndex[cd.Name] = len(t.cols) - 1
	}

	return nil
}

func (t *table) dropColumn(name spansql.ID) *status.Status {
	// Only permit dropping non-key columns that aren't part of a secondary index.
	// We don't support indexes, so only check that it isn't part of the primary key.

	t.mu.Lock()
	defer t.mu.Unlock()

	ci, ok := t.colIndex[name]
	if !ok {
		// TODO: What's the right response code?
		return status.Newf(codes.InvalidArgument, "unknown column %q", name)
	}
	if ci < t.pkCols {
		// TODO: What's the right response code?
		return status.Newf(codes.InvalidArgument, "can't drop primary key column %q", name)
	}

	// Remove from cols and colIndex, and renumber colIndex and origIndex.
	t.cols = append(t.cols[:ci], t.cols[ci+1:]...)
	delete(t.colIndex, name)
	for i, col := range t.cols {
		t.colIndex[col.Name] = i
	}
	pre := t.origIndex[name]
	delete(t.origIndex, name)
	for n, i := range t.origIndex {
		if i > pre {
			t.origIndex[n]--
		}
	}

	// Drop data.
	for i := range t.rows {
		t.rows[i] = append(t.rows[i][:ci], t.rows[i][ci+1:]...)
	}

	return nil
}

func (t *table) alterColumn(alt spansql.AlterColumn) *status.Status {
	// Supported changes here are:
	//	Add NOT NULL to a non-key column, excluding ARRAY columns.
	//	Remove NOT NULL from a non-key column.
	//	Change a STRING column to a BYTES column or a BYTES column to a STRING column.
	//	Increase or decrease the length limit for a STRING or BYTES type (including to MAX).
	//	Enable or disable commit timestamps in value and primary key columns.
	// https://cloud.google.com/spanner/docs/schema-updates#supported-updates

	// TODO: codes.InvalidArgument is used throughout here for reporting errors,
	// but that has not been validated against the real Spanner.

	sct, ok := alt.Alteration.(spansql.SetColumnType)
	if !ok {
		return status.Newf(codes.InvalidArgument, "unsupported ALTER COLUMN %s", alt.SQL())
	}

	t.mu.Lock()
	defer t.mu.Unlock()

	ci, ok := t.colIndex[alt.Name]
	if !ok {
		return status.Newf(codes.InvalidArgument, "unknown column %q", alt.Name)
	}

	oldT, newT := t.cols[ci].Type, sct.Type
	stringOrBytes := func(bt spansql.TypeBase) bool { return bt == spansql.String || bt == spansql.Bytes }

	// First phase: Check the validity of the change.
	// TODO: Don't permit changes to allow commit timestamps.
	if !t.cols[ci].NotNull && sct.NotNull {
		// Adding NOT NULL is not permitted for primary key columns or array typed columns.
		if ci < t.pkCols {
			return status.Newf(codes.InvalidArgument, "cannot set NOT NULL on primary key column %q", alt.Name)
		}
		if oldT.Array {
			return status.Newf(codes.InvalidArgument, "cannot set NOT NULL on array-typed column %q", alt.Name)
		}
		// Validate that there are no NULL values.
		for _, row := range t.rows {
			if row[ci] == nil {
				return status.Newf(codes.InvalidArgument, "cannot set NOT NULL on column %q that contains NULL values", alt.Name)
			}
		}
	}
	var conv func(x interface{}) interface{}
	if stringOrBytes(oldT.Base) && stringOrBytes(newT.Base) && !oldT.Array && !newT.Array {
		// Change between STRING and BYTES is fine, as is increasing/decreasing the length limit.
		// TODO: This should permit array conversions too.
		// TODO: Validate data; length limit changes should be rejected if they'd lead to data loss, for instance.
		if oldT.Base == spansql.Bytes && newT.Base == spansql.String {
			conv = func(x interface{}) interface{} { return string(x.([]byte)) }
		} else if oldT.Base == spansql.String && newT.Base == spansql.Bytes {
			conv = func(x interface{}) interface{} { return []byte(x.(string)) }
		}
	} else if oldT == newT {
		// Same type; only NOT NULL changes.
	} else { // TODO: Support other alterations.
		return status.Newf(codes.InvalidArgument, "unsupported ALTER COLUMN %s", alt.SQL())
	}

	// Second phase: Make type transformations.
	t.cols[ci].NotNull = sct.NotNull
	t.cols[ci].Type = newT
	if conv != nil {
		for _, row := range t.rows {
			if row[ci] != nil { // NULL stays as NULL.
				row[ci] = conv(row[ci])
			}
		}
	}
	return nil
}

func (t *table) addRowDeletionPolicy(ard spansql.AddRowDeletionPolicy) *status.Status {
	_, ok := t.colIndex[ard.RowDeletionPolicy.Column]
	if !ok {
		return status.Newf(codes.InvalidArgument, "unknown column %q", ard.RowDeletionPolicy.Column)
	}
	if t.rdw != nil {
		return status.New(codes.InvalidArgument, "table already has a row deletion policy")
	}
	t.rdw = &ard.RowDeletionPolicy
	return nil
}

func (t *table) replaceRowDeletionPolicy(ard spansql.ReplaceRowDeletionPolicy) *status.Status {
	_, ok := t.colIndex[ard.RowDeletionPolicy.Column]
	if !ok {
		return status.Newf(codes.InvalidArgument, "unknown column %q", ard.RowDeletionPolicy.Column)
	}
	if t.rdw == nil {
		return status.New(codes.InvalidArgument, "table does not have a row deletion policy")
	}
	t.rdw = &ard.RowDeletionPolicy
	return nil
}

func (t *table) dropRowDeletionPolicy(ard spansql.DropRowDeletionPolicy) *status.Status {
	if t.rdw == nil {
		return status.New(codes.InvalidArgument, "table does not have a row deletion policy")
	}
	t.rdw = nil
	return nil
}

func (t *table) insertRow(rowNum int, r row) {
	t.rows = append(t.rows, nil)
	copy(t.rows[rowNum+1:], t.rows[rowNum:])
	t.rows[rowNum] = r
}

// findRange finds the rows included in the key range,
// reporting it as a half-open interval.
// r.startKey and r.endKey should be populated.
func (t *table) findRange(r *keyRange) (int, int) {
	// startRow is the first row matching the range.
	startRow := sort.Search(len(t.rows), func(i int) bool {
		return rowCmp(r.startKey, t.rows[i][:t.pkCols], t.pkDesc) <= 0
	})
	if startRow == len(t.rows) {
		return startRow, startRow
	}
	if !r.startClosed && rowCmp(r.startKey, t.rows[startRow][:t.pkCols], t.pkDesc) == 0 {
		startRow++
	}

	// endRow is one more than the last row matching the range.
	endRow := sort.Search(len(t.rows), func(i int) bool {
		return rowCmp(r.endKey, t.rows[i][:t.pkCols], t.pkDesc) < 0
	})
	if !r.endClosed && rowCmp(r.endKey, t.rows[endRow-1][:t.pkCols], t.pkDesc) == 0 {
		endRow--
	}

	return startRow, endRow
}

// colIndexes returns the indexes for the named columns.
func (t *table) colIndexes(cols []spansql.ID) ([]int, error) {
	var is []int
	for _, col := range cols {
		i, ok := t.colIndex[col]
		if !ok {
			return nil, status.Errorf(codes.InvalidArgument, "column %s not in table", col)
		}
		is = append(is, i)
	}
	return is, nil
}

// primaryKey constructs the internal representation of a primary key.
// The list of given values must be in 1:1 correspondence with the primary key of the table.
func (t *table) primaryKey(values []*structpb.Value) ([]interface{}, error) {
	if len(values) != t.pkCols {
		return nil, status.Errorf(codes.InvalidArgument, "primary key length mismatch: got %d values, table has %d", len(values), t.pkCols)
	}
	return t.primaryKeyPrefix(values)
}

// primaryKeyPrefix constructs the internal representation of a primary key prefix.
func (t *table) primaryKeyPrefix(values []*structpb.Value) ([]interface{}, error) {
	if len(values) > t.pkCols {
		return nil, status.Errorf(codes.InvalidArgument, "primary key length too long: got %d values, table has %d", len(values), t.pkCols)
	}

	var pk []interface{}
	for i, value := range values {
		v, err := valForType(value, t.cols[i].Type)
		if err != nil {
			return nil, err
		}
		pk = append(pk, v)
	}
	return pk, nil
}

// rowForPK returns the index of t.rows that holds the row for the given primary key, and true.
// If the given primary key isn't found, it returns the row that should hold it, and false.
func (t *table) rowForPK(pk []interface{}) (row int, found bool) {
	if len(pk) != t.pkCols {
		panic(fmt.Sprintf("primary key length mismatch: got %d values, table has %d", len(pk), t.pkCols))
	}

	i := sort.Search(len(t.rows), func(i int) bool {
		return rowCmp(pk, t.rows[i][:t.pkCols], t.pkDesc) <= 0
	})
	if i == len(t.rows) {
		return i, false
	}
	return i, rowEqual(pk, t.rows[i][:t.pkCols])
}

// rowCmp compares two rows, returning -1/0/+1.
// The desc arg indicates whether each column is in a descending order.
// This is used for primary key matching and so doesn't support array/struct types.
// a is permitted to be shorter than b.
func rowCmp(a, b []interface{}, desc []bool) int {
	for i := 0; i < len(a); i++ {
		if cmp := compareVals(a[i], b[i]); cmp != 0 {
			if desc[i] {
				cmp = -cmp
			}
			return cmp
		}
	}
	return 0
}

// rowEqual reports whether two rows are equal.
// This doesn't support array/struct types.
func rowEqual(a, b []interface{}) bool {
	for i := 0; i < len(a); i++ {
		if compareVals(a[i], b[i]) != 0 {
			return false
		}
	}
	return true
}

// valForType converts a value from its RPC form into its internal representation.
func valForType(v *structpb.Value, t spansql.Type) (interface{}, error) {
	if _, ok := v.Kind.(*structpb.Value_NullValue); ok {
		return nil, nil
	}

	if lv, ok := v.Kind.(*structpb.Value_ListValue); ok && t.Array {
		et := t // element type
		et.Array = false

		// Construct the non-nil slice for the list.
		arr := make([]interface{}, 0, len(lv.ListValue.Values))
		for _, v := range lv.ListValue.Values {
			x, err := valForType(v, et)
			if err != nil {
				return nil, err
			}
			arr = append(arr, x)
		}
		return arr, nil
	}

	switch t.Base {
	case spansql.Bool:
		bv, ok := v.Kind.(*structpb.Value_BoolValue)
		if ok {
			return bv.BoolValue, nil
		}
	case spansql.Int64:
		// The Spanner protocol encodes int64 as a decimal string.
		sv, ok := v.Kind.(*structpb.Value_StringValue)
		if ok {
			x, err := strconv.ParseInt(sv.StringValue, 10, 64)
			if err != nil {
				return nil, fmt.Errorf("bad int64 string %q: %v", sv.StringValue, err)
			}
			return x, nil
		}
	case spansql.Float64:
		nv, ok := v.Kind.(*structpb.Value_NumberValue)
		if ok {
			return nv.NumberValue, nil
		}
	case spansql.String:
		sv, ok := v.Kind.(*structpb.Value_StringValue)
		if ok {
			return sv.StringValue, nil
		}
	case spansql.Bytes:
		sv, ok := v.Kind.(*structpb.Value_StringValue)
		if ok {
			// The Spanner protocol encodes BYTES in base64.
			return base64.StdEncoding.DecodeString(sv.StringValue)
		}
	case spansql.Date:
		// The Spanner protocol encodes DATE in RFC 3339 date format.
		sv, ok := v.Kind.(*structpb.Value_StringValue)
		if ok {
			s := sv.StringValue
			d, err := parseAsDate(s)
			if err != nil {
				return nil, fmt.Errorf("bad DATE string %q: %v", s, err)
			}
			return d, nil
		}
	case spansql.Timestamp:
		// The Spanner protocol encodes TIMESTAMP in RFC 3339 timestamp format with zone Z.
		sv, ok := v.Kind.(*structpb.Value_StringValue)
		if ok {
			s := sv.StringValue
			if strings.ToLower(s) == "spanner.commit_timestamp()" {
				return commitTimestampSentinel, nil
			}
			t, err := parseAsTimestamp(s)
			if err != nil {
				return nil, fmt.Errorf("bad TIMESTAMP string %q: %v", s, err)
			}
			return t, nil
		}
	}
	return nil, fmt.Errorf("unsupported inserting value kind %T into column of type %s", v.Kind, t.SQL())
}

type keyRange struct {
	start, end             *structpb.ListValue
	startClosed, endClosed bool

	// These are populated during an operation
	// when we know what table this keyRange applies to.
	startKey, endKey []interface{}
}

func (r *keyRange) String() string {
	var sb bytes.Buffer // TODO: Switch to strings.Builder when we drop support for Go 1.9.
	if r.startClosed {
		sb.WriteString("[")
	} else {
		sb.WriteString("(")
	}
	fmt.Fprintf(&sb, "%v,%v", r.startKey, r.endKey)
	if r.endClosed {
		sb.WriteString("]")
	} else {
		sb.WriteString(")")
	}
	return sb.String()
}

type keyRangeList []*keyRange

// Execute runs a DML statement.
// It returns the number of affected rows.
func (d *database) Execute(stmt spansql.DMLStmt, params queryParams) (int, error) { // TODO: return *status.Status instead?
	switch stmt := stmt.(type) {
	default:
		return 0, status.Errorf(codes.Unimplemented, "unhandled DML statement type %T", stmt)
	case *spansql.Delete:
		t, err := d.table(stmt.Table)
		if err != nil {
			return 0, err
		}

		t.mu.Lock()
		defer t.mu.Unlock()

		n := 0
		for i := 0; i < len(t.rows); {
			ec := evalContext{
				cols:   t.cols,
				row:    t.rows[i],
				params: params,
			}
			b, err := ec.evalBoolExpr(stmt.Where)
			if err != nil {
				return 0, err
			}
			if b != nil && *b {
				copy(t.rows[i:], t.rows[i+1:])
				t.rows = t.rows[:len(t.rows)-1]
				n++
				continue
			}
			i++
		}
		return n, nil
	case *spansql.Update:
		t, err := d.table(stmt.Table)
		if err != nil {
			return 0, err
		}

		t.mu.Lock()
		defer t.mu.Unlock()

		ec := evalContext{
			cols:   t.cols,
			params: params,
		}

		// Build parallel slices of destination column index and expressions to evaluate.
		var dstIndex []int
		var expr []spansql.Expr
		for _, ui := range stmt.Items {
			i, err := ec.resolveColumnIndex(ui.Column)
			if err != nil {
				return 0, err
			}
			// TODO: Enforce "A column can appear only once in the SET clause.".
			if i < t.pkCols {
				return 0, status.Errorf(codes.InvalidArgument, "cannot update primary key %s", ui.Column)
			}
			dstIndex = append(dstIndex, i)
			expr = append(expr, ui.Value)
		}

		n := 0
		values := make(row, len(stmt.Items)) // scratch space for new values
		for i := 0; i < len(t.rows); i++ {
			ec.row = t.rows[i]
			b, err := ec.evalBoolExpr(stmt.Where)
			if err != nil {
				return 0, err
			}
			if b != nil && *b {
				// Compute every update item.
				for j := range dstIndex {
					if expr[j] == nil { // DEFAULT
						values[j] = nil
						continue
					}
					v, err := ec.evalExpr(expr[j])
					if err != nil {
						return 0, err
					}
					values[j] = v
				}
				// Write them to the row.
				for j, v := range values {
					t.rows[i][dstIndex[j]] = v
				}
				n++
			}
		}
		return n, nil
	}
}

func parseAsDate(s string) (civil.Date, error) { return civil.ParseDate(s) }
func parseAsTimestamp(s string) (time.Time, error) {
	return time.Parse("2006-01-02T15:04:05.999999999Z", s)
}