tantivy/sstable

SSTable

The tantivy-sstable crate is yet another sstable crate.

It has been designed to be used in quickwit:

• as an alternative to the default tantivy fst dictionary.
• as a way to store the column index for dynamic fast fields.

The benefit compared to the fst crate is locality. Searching a key in the fst crate requires downloading the entire dictionary.

Once the sstable index is downloaded, running a get in the sstable crate only requires a single fetch.

Right now, the block index and the default block size have been thought for quickwit, and the performance of a get is very bad.

Sorted strings?

SSTable stands for Sorted String Table. Strings have to be insert in sorted order.

That sorted order is used in different ways:

• it makes gets and streaming ranges of keys possible.
• it allows incremental encoding of the keys
• the front compression is leveraged to optimize the intersection with an automaton

On disk format

Overview of the SSTable format. Unless noted otherwise, numbers are little-endian.

SSTable

+-------+-------+-----+--------+
| Block | Block | ... | Footer |
+-------+-------+-----+--------+
|----( # of blocks)---|

• Block(SSTBlock): list of independent block, terminated by a single empty block.
• Footer(SSTFooter)

SSTBlock

+----------+----------+--------+-------+-------+-----+
| BlockLen | Compress | Values | Delta | Delta | ... |
+----------+----------+--------+-------+-------+-----+
                      |        |----( # of deltas)---|
                      |------(maybe compressed)------|

• BlockLen(u32): length of the block, including the compress byte.
• Compress(u8): indicate whether block is compressed. 0 if not compressed, 1 if compressed.
• Values: an application defined format storing a sequence of value, capable of determining it own length
• Delta

Delta

+---------+--------+
| KeepAdd | Suffix |
+---------+--------+

• KeepAdd
• Suffix: KeepAdd.add bytes of key suffix

KeepAdd

KeepAdd can be represented in two different representation, a very compact 1byte one which is enough for most usage, and a longer variable-len one when required

When keep < 16 and add < 16

+-----+------+
| Add | Keep |
+-----+------+

• Add(u4): number of bytes to push
• Keep(u4): number of bytes to pop

Otherwise:

+------+------+-----+
| 0x01 | Keep | Add |
+------+------+-----+

• Add(VInt): number of bytes to push
• Keep(VInt): number of bytes to pop

Note: as the SSTable does not support redundant keys, there is no ambiguity between both representation. Add is always guaranteed to be non-zero, except for the very first key of an SSTable, where Keep is guaranteed to be zero.

SSTFooter

+-----+----------------+-------------+-------------+---------+---------+
| Fst | BlockAddrStore | StoreOffset | IndexOffset | NumTerm | Version |
+-----+----------------+-------------+-------------+---------+---------+

• Fst(Fst): finite state transducer mapping keys to a block number
• BlockAddrStore(BlockAddrStore): store mapping a block number to its BlockAddr
• StoreOffset(u64): Offset to start of the BlockAddrStore. If zero, see the SingleBlockSStable section
• IndexOffset(u64): Offset to the start of the SSTFooter
• NumTerm(u64): number of terms in the sstable
• Version(u32): Currently equal to 3

Fst

Fst is in the format of tantivy_fst

BlockAddrStore

+---------+-----------+-----------+-----+-----------+-----------+-----+ | MetaLen | BlockMeta | BlockMeta | ... | BlockData | BlockData | ... | +---------+-----------+-----------+-----+-----------+-----------+-----+ |---------(N blocks)----------|---------(N blocks)----------|

• MetaLen(u64): length of the BlockMeta section
• BlockMeta(BlockAddrBlockMetadata): metadata to seek through BlockData
• BlockData(CompactedBlockAddr): bitpacked per block metadata

BlockAddrBlockMetadata

+--------+------------+--------------+------------+--------------+-------------------+-----------------+----------+ | Offset | RangeStart | FirstOrdinal | RangeSlope | OrdinalSlope | FirstOrdinalNBits | RangeStartNBits | BlockLen | +--------+------------+--------------+------------+--------------+-------------------+-----------------+----------+

• Offset(u64): offset of the corresponding BlockData in the datastream
• RangeStart(u64): the start position of the first block
• FirstOrdinal(u64): the first ordinal of the first block
• RangeSlope(u32): slope predicted for start range evolution (see computation in BlockData)
• OrdinalSlope(u64): slope predicted for first ordinal evolution (see computation in BlockData)
• FirstOrdinalNBits(u8): number of bits per ordinal in datastream (see computation in BlockData)
• RangeStartNBits(u8): number of bits per range start in datastream (see computation in BlockData)

BlockData

+-----------------+-------------------+---------------+ | RangeStartDelta | FirstOrdinalDelta | FinalRangeEnd | +-----------------+-------------------+---------------+ |------(BlockLen repetitions)---------|

• RangeStartDelta(var): RangeStartNBits bits of little endian number. See below for decoding
• FirstOrdinalDelta(var): FirstOrdinalNBits bits of little endian number. See below for decoding
• FinalRangeEnd(var): RangeStartNBits bits of integer. See below for decoding

converting a BlockData of index Index and a BlockAddrBlockMetadata to an actual block address is done as follow: range_prediction := RangeStart + Index * RangeSlop; range_derivation := RangeStartDelta - (1 << (RangeStartNBits-1)); range_start := range_prediction + range_derivation

The same computation can be done for ordinal.

Note that range_derivation can take negative value. RangeStartDelta is just its translation to a positive range.

SingleBlockSStable

The format used for the index is meant to be compact, however it has a constant cost of around 70 bytes, which isn't negligible for a table containing very few keys. To limit the impact of that constant cost, single block sstable omit the Fst and BlockAddrStore from their index. Instead a block with first ordinal of 0, range start of 0 and range end of IndexOffset is implicitly used for every operations.