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iterator.md

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Iterator

BMDB stores its keys in byte-sorted order within a bucket. This makes sequential iteration over these keys extremely fast.

Prefix scans

To iterate over a key prefix, we can use PrefixScanEntries function, and the parameters offsetNum and limitNum constrain the number of entries returned :

if err := db.View(
    func(tx *bmdb.Tx) error {
        prefix := []byte("user_")
        bucket := "user_list"
        // Constrain 100 entries returned 
        if keys, values, err := tx.PrefixScanEntries(bucket, prefix, "", 25, 100, true, true); err != nil {
            return err
        } else {
            for k := range keys {
                fmt.Println(string(keys[k]), string(values[k]))
            }
        }
        return nil
    }); err != nil {
        log.Fatal(err)
}

Prefix search scans

To iterate over a key prefix with search by regular expression on a second part of key without prefix, we can use PrefixScanEntries function when using the reg parameter,the parameters offsetNum, limitNum constrain the number of entries returned :

if err := db.View(
    func(tx *bmdb.Tx) error {
        prefix := []byte("user_")
        reg := "username"
        bucket := "user_list"
        // Constrain 100 entries returned 
        if keys, values, err := tx.PrefixScanEntries(bucket, prefix, reg, 25, 100, true, true); err != nil {
            return err
        } else {
            for k := range keys {
                fmt.Println(string(keys[k]), string(values[k]))
            }
        }
        return nil
    }); err != nil {
        log.Fatal(err)
}

Range scans

To scan over a range, we can use RangeScanEntries function. For example：

if err := db.View(
    func(tx *bmdb.Tx) error {
        // Assume key from user_0000000 to user_9999999.
        // Query a specific user key range like this.
        start := []byte("user_0010001")
        end := []byte("user_0010010")
        bucket := "user_list"
        if keys, values, err := tx.RangeScanEntries(bucket, start, end, true, true); err != nil {
            return err
        } else {
            for k := range keys {
                fmt.Println(string(keys[k]), string(values[k]))
            }
        }
        return nil
    }); err != nil {
    log.Fatal(err)
}

Get all

To scan all keys and values of the bucket stored, we can use GetAll function. For example:

if err := db.View(
    func(tx *bmdb.Tx) error {
        bucket := "user_list"
        entries, err := tx.GetAll(bucket)
        if err != nil {
            return err
        }

        for _, entry := range entries {
            fmt.Println(string(entry.Key),string(entry.Value))
        }

        return nil
    }); err != nil {
    log.Println(err)
}

iterator

The option parameter Reverse that determines whether the iterator is forward or Reverse. The current version does not support the iterator for HintBPTSparseIdxMode.

ForwardReverse
 
tx, err := db.Begin(false)
iterator := bmdb.NewIterator(tx, bucket, bmdb.IteratorOptions{Reverse: false})
i := 0
for i < 10 {
    ok, err := iterator.SetNext()
    fmt.Println("ok, err", ok, err)
    fmt.Println("Key: ", string(iterator.Entry().Key))
    fmt.Println("Value: ", string(iterator.Entry().Value))
    fmt.Println()
    i++
}
err = tx.Commit()
if err != nil {
    panic(err)
}
 
tx, err := db.Begin(false)
iterator := bmdb.NewIterator(tx, bucket, bmdb.IteratorOptions{Reverse: true})
i := 0
for i < 10 {
    ok, err := iterator.SetNext()
    fmt.Println("ok, err", ok, err)
    fmt.Println("Key: ", string(iterator.Entry().Key))
    fmt.Println("Value: ", string(iterator.Entry().Value))
    fmt.Println()
    i++
}
err = tx.Commit()
if err != nil {
    panic(err)
}