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从头实现数据库-10

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二级索引

在本章中，我们将向数据库添加额外的索引（也称为二级索引）。查询将不再局限于主键。
 

索引定义

在表定义中添加了索引和索引前缀字段。与表本身一样，每个索引在KV存储中都分配了一个键前缀。
// table definition
type TableDef struct {
    // user defined
    Name    string
    Types   []uint32 // column types
    Cols    []string // column names
    PKeys   int      // the first `PKeys` columns are the primary key
    Indexes [][]string
    // auto-assigned B-tree key prefixes for different tables/indexes
    Prefix        uint32
    IndexPrefixes []uint32
}
要通过索引找到行，索引必须包含主键的副本。我们将通过将主键列附加到索引中来实现这一点；这也使索引键唯一，这是B树查找代码的假设。
func checkIndexKeys(tdef *TableDef, index []string) ([]string, error) {
    icols := map[string]bool{}
    for _, c := range index {
        // check the index columns
        // omitted...
        icols[c] = true
    }
    // add the primary key to the index
    for _, c := range tdef.Cols[:tdef.PKeys] {
        if !icols[c] {
            index = append(index, c)
        }
    }
    assert(len(index) < len(tdef.Cols))
    return index, nil
}

func colIndex(tdef *TableDef, col string) int {
    for i, c := range tdef.Cols {
        if c == col {
            return i
        }
    }
    return -1
}
在创建新表之前，会检查索引并附加主键。
func tableDefCheck(tdef *TableDef) error {
    // verify the table definition
    // omitted...

    // verify the indexes
    for i, index := range tdef.Indexes {
        index, err := checkIndexKeys(tdef, index)
        if err != nil {
            return err
        }
        tdef.Indexes[i] = index
    }
    return nil
}
创建新表时分配多个键前缀。
 
// create a new table
func (db *DB) TableNew(tdef *TableDef) error {
    if err := tableDefCheck(tdef); err != nil {
        return err
    }

    // check the existing table
    // omited...

    // allocate new prefixes
    tdef.Prefix = /* omited... */
    for i := range tdef.Indexes {
        prefix := tdef.Prefix + 1 + uint32(i)
        tdef.IndexPrefixes = append(tdef.IndexPrefixes, prefix)
    }

    // update the next prefix
    ntree := 1 + uint32(len(tdef.Indexes))
    binary.LittleEndian.PutUint32(meta.Get("val").Str, tdef.Prefix+ntree)
    _, err = dbUpdate(db, TDEF_META, *meta, 0)
    if err != nil {
        return err
    }

    // store the definition
    // omited...
}
 

维护索引

在更新行后，我们需要从索引中删除旧行。B树接口被修改为返回更新前的值。
 
type InsertReq struct {
    tree *BTree
    // out
    Added   bool   // added a new key
    Updated bool   // added a new key or an old key was changed
    Old     []byte // the value before the update
    // in
    Key  []byte
    Val  []byte
    Mode int
}

type DeleteReq struct {
    tree *BTree
    // in
    Key []byte
    // out
    Old []byte
}

func (tree *BTree) InsertEx(req *InsertReq)
func (tree *BTree) DeleteEx(req *DeleteReq)
下面是添加或删除索引中记录的函数。在这里，我们遇到了一个问题：更新具有二级索引的表涉及KV 存储中的多个键，这应该以原子方式进行。我们将在以后的章节中解决这个问题。
 
 
const (
    INDEX_ADD = 1
    INDEX_DEL = 2
)

// maintain indexes after a record is added or removed
func indexOp(db *DB, tdef *TableDef, rec Record, op int) {
    key := make([]byte, 0, 256)
    irec := make([]Value, len(tdef.Cols))
    for i, index := range tdef.Indexes {
        // the indexed key
        for j, c := range index {
            irec[j] = *rec.Get(c)
        }
        // update the KV store
        key = encodeKey(key[:0], tdef.IndexPrefixes[i], irec[:len(index)])
        done, err := false, error(nil)
        switch op {
        case INDEX_ADD:
            done, err = db.kv.Update(&InsertReq{Key: key})
        case INDEX_DEL:
            done, err = db.kv.Del(&DeleteReq{Key: key})
        default:
            panic("what?")
        }
        assert(err == nil) // XXX: will fix this in later chapters
        assert(done)
    }
}
更新或删除行后维护索引：
// add a row to the table
func dbUpdate(db *DB, tdef *TableDef, rec Record, mode int) (bool, error) {
    // omitted...

    req := InsertReq{Key: key, Val: val, Mode: mode}
    added, err := db.kv.Update(&req)
    if err != nil || !req.Updated || len(tdef.Indexes) == 0 {
        return added, err
    }

    // maintain indexes
    if req.Updated && !req.Added {
        decodeValues(req.Old, values[tdef.PKeys:]) // get the old row
        indexOp(db, tdef, Record{tdef.Cols, values}, INDEX_DEL)
    }
    if req.Updated {
        indexOp(db, tdef, rec, INDEX_ADD)
    }
    return added, nil
}
// delete a record by its primary key
func dbDelete(db *DB, tdef *TableDef, rec Record) (bool, error) {
    // omitted...

    deleted, err := db.kv.Del(&req)
    if err != nil || !deleted || len(tdef.Indexes) == 0 {
        return deleted, err
    }

    // maintain indexes
    if deleted {
        // likewise...
    }
    return true, nil
}

使用二级索引

选择索引

我们还将使用索引的前缀执行范围查询。例如，我们可以在包含前缀【a】的索引【a, b, c】上执行x < a AND a < y。选择索引只是通过输入前缀匹配列。主键在二级索引之前考虑。
func findIndex(tdef *TableDef, keys []string) (int, error) {
    pk := tdef.Cols[:tdef.PKeys]
    if isPrefix(pk, keys) {
        // use the primary key.
        // also works for full table scans without a key.
        return -1, nil
    }

    // find a suitable index
    winner := -2
    for i, index := range tdef.Indexes {
        if !isPrefix(index, keys) {
            continue
        }
        if winner == -2 || len(index) < len(tdef.Indexes[winner]) {
            winner = i
        }
    }
    if winner == -2 {
        return -2, fmt.Errorf("no index found")
    }
    return winner, nil
}

func isPrefix(long []string, short []string) bool {
    if len(long) < len(short) {
        return false
    }
    for i, c := range short {
        if long[i] != c {
            return false
        }
    }
    return true
}

编码索引前缀

如果输入键使用索引的前缀而不是完整的索引，我们可能需要编码额外的列。例如，对于带有索引【a，b】的查询v1 < a，我们不能使用【v1】 < key作为底层B树查询，因为任何键【v1，v2】都满足【v1】 < 【v1，v2】，而违反v1 < a。
相反，在这种情况下，我们可以使用【v1，MAX】 < key，其中MAX是列b的最大可能值。以下是编码带有额外列的部分查询键的函数。
// The range key can be a prefix of the index key,
// we may have to encode missing columns to make the comparison work.
func encodeKeyPartial(
    out []byte, prefix uint32, values []Value,
    tdef *TableDef, keys []string, cmp int,
) []byte {
    out = encodeKey(out, prefix, values)

    // Encode the missing columns as either minimum or maximum values,
    // depending on the comparison operator.
    // 1. The empty string is lower than all possible value encodings,
    //    thus we don't need to add anything for CMP_LT and CMP_GE.
    // 2. The maximum encodings are all 0xff bytes.
    max := cmp == CMP_GT || cmp == CMP_LE
loop:
    for i := len(values); max && i < len(keys); i++ {
        switch tdef.Types[colIndex(tdef, keys[i])] {
        case TYPE_BYTES:
            out = append(out, 0xff)
            break loop // stops here since no string encoding starts with 0xff
        case TYPE_INT64:
            out = append(out, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff)
        default:
            panic("what?")
        }
    }
    return out
}
对于 int64 类型，最大值编码为所有 0xff 字节。问题是字符串没有最大值。我们可以做的就是使用"\xff" 作为 "伪最大字符串值" 的编码，并将正常字符串编码更改为不以 "\xff" 开头。
如果一个字符串的第一字节是"\xff"或"\xfe"，那么它就会被"\xfe"字节转义。因此，所有字符串编码都小于"\xff"。
// 1. strings are encoded as null-terminated strings,
//    escape the null byte so that strings contain no null byte.
// 2. "\xff" represents the highest order in key comparisons,
//    also escape the first byte if it's 0xff.
func escapeString(in []byte) []byte {
    // omitted...

    pos := 0
    if len(in) > 0 && in[0] >= 0xfe {
        out[0] = 0xfe
        out[1] = in[0]
        pos += 2
        in = in[1:]
    }

    // omitted...
    return out
}

通过索引检索行

索引键包含所有主键列，因此我们可以找到完整的行。Scanner类型现在知道选择了哪个索引。
// the iterator for range queries
type Scanner struct {
    // omitted...
    db      *DB
    tdef    *TableDef
    indexNo int    // -1: use the primary key; >= 0: use an index
    iter    *BIter // the underlying B-tree iterator
    keyEnd  []byte // the encoded Key2
}
// fetch the current row
func (sc *Scanner) Deref(rec *Record) {
    assert(sc.Valid())

    tdef := sc.tdef
    rec.Cols = tdef.Cols
    rec.Vals = rec.Vals[:0]
    key, val := sc.iter.Deref()

    if sc.indexNo < 0 {
        // primary key, decode the KV pair
        // omitted...
    } else {
        // secondary index
        // The "value" part of the KV store is not used by indexes
        assert(len(val) == 0)

        // decode the primary key first
        index := tdef.Indexes[sc.indexNo]
        ival := make([]Value, len(index))
        for i, c := range index {
            ival[i].Type = tdef.Types[colIndex(tdef, c)]
        }
        decodeValues(key[4:], ival)
        icol := Record{index, ival}

        // fetch the row by the primary key
        rec.Cols = tdef.Cols[:tdef.PKeys]
        for _, c := range rec.Cols {
            rec.Vals = append(rec.Vals, *icol.Get(c))
        }
        // TODO: skip this if the index contains all the columns
        ok, err := dbGet(sc.db, tdef, rec)
        assert(ok && err == nil)
    }
}

将所有部件组合在一起

修改后的 dbScan 函数使用二级索引。现在还可以通过索引前缀进行范围查询。它还可以扫描整个表，而不需要任何键（如果没有列，则选择主键）。
func dbScan(db *DB, tdef *TableDef, req *Scanner) error {
    // sanity checks
    // omitted...

    // select an index
    indexNo, err := findIndex(tdef, req.Key1.Cols)
    if err != nil {
        return err
    }
    index, prefix := tdef.Cols[:tdef.PKeys], tdef.Prefix
    if indexNo >= 0 {
        index, prefix = tdef.Indexes[indexNo], tdef.IndexPrefixes[indexNo]
    }

    req.db = db
    req.tdef = tdef
    req.indexNo = indexNo

    // seek to the start key
    keyStart := encodeKeyPartial(
        nil, prefix, req.Key1.Vals, tdef, index, req.Cmp1)
    req.keyEnd = encodeKeyPartial(
        nil, prefix, req.Key2.Vals, tdef, index, req.Cmp2)
    req.iter = db.kv.tree.Seek(keyStart, req.Cmp1)
    return nil
}
我们实现了关系数据库的一些主要功能：表、范围查询和二级索引。我们可以开始向数据库添加更多功能和查询语言。然而，还有一些主要方面仍然缺失：事务和并发性，这些将在以后的章节中进行探索。

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