[tarantool-patches] Re: [PATCH v1 1/1] rfc: describe a Tarantool JSON indexes

[Kirill Shcherbatov]

Hi! Thanks for so detailed and informative criticism!
I've updated RFC. I've tried to be more accurate with details and provide detailed explanations. 
https://github.com/tarantool/tarantool/blob/a78d21282d5cb9033699d3cb50ead91d01dffbb4/doc/rfc/1012-json-indexes.md

=====================================================

# Tarantool JSON Indexes

* **Status**: In progress
* **Start date**: 26-07-2018
* **Authors**: Kirill Shcherbatov @kshcherbatov kshcherbatov at tarantool.org
* **Issues**: [#1012](https://github.com/tarantool/tarantool/issues/1012)

## Summary
Tarantool *JSON Indexes* is a feature that allows to index space's tuples that have consistent document-type complex structure.


## Background and motivation
The box.index submodule provides read-only access for index definitions and index keys. Indexes are contained in box.space.space-name.index array within each space object. They provide an API for ordered iteration over tuples. This API is a direct binding to corresponding methods of index objects of type box.index in the storage engine.
```
s:create_index('primary', {parts = {1, 'unsigned'}, {2, 'str'}})
```

Sometimes tuple space data have complex but consistent document structure.
```
s:insert{1, {town = 'NY', name = {first = 'Richard', second = 'Feynman'}}}
s:insert{2, {town = 'NY', name = {first = 'Ayn', second = 'Rand'}}}
```
Building index on document fields would be attractive feature that allows to search and iterate over such data.
```
s:create_index('name'', { parts = {{ "name.first", "str" }, {"name.last", "str"}}})
```

Fields having complex document structure should have 'map' type in format if specified.


## Detailed design
All data in Tarantool stored in atomic database objects called *tuples*. They consist of payload(user data represented as *msgpack*) and extra information that describe how does database should operate with it.
```
  [[ Img 0.1 - Tarantool tuple structure ]]

                           +----------------------+-----------------------+-------------+
  tuple_begin, ..., raw =  |      extra_size      | offset N ... offset 1 | MessagePack |
  |                        +----------------------+-----------------------+-------------+
  |                                                                       ^
  +-----------------------------------------------------------------------data_offset
```
Indexed information should be accessible really fast. In case of regular indexes on fields it comes possible with *field_map* - data offsets stored before user payload in each tuple. Metadata cells that are used to store those offsets *offset_slot* are not same for different *formats*(aggregated information of all indexes describing specific space). Without loss of generality this mean that tuples constructed at the moment 'A' when only PK index was registered have another *format* than the tuples constructed at the moment 'B' after new Index "name" was added.

The concept idea with *offset_slot*s could be used for JSON Indexes too.

JSON-path definitely would be a part of *key_part* structure. As we have already noted, *offset_slots* are different for formats of different generations, so we should find an universal and fast way to obtain it by JSON. As code that access tuple data by index is a really hot, the performance is our priority problem.
We already have *tuple_field_raw_by_path* that follows JSON path in tuple and returns pointer to requested field. Sometimes fields that nested in JSON-structured document may have coinciding path part:
e.g.:
```
s:create_index('name'', { parts = {{ "name.first", "str" }, {"name.last", "str"}}})
s:create_index('name'', { parts = {{ "town", "str" }, {"name.last", "str"}}})
```
We don't like to parse coinciding suffixes multiple time for same tuples.

### 1.1. JSON-path tree in tuple format
Keeping in mind all challenges we suppose to introduce tree-based structure in *tuple_format* that contain JSON-path lexemes and has allocated tuple *offset_slots* as leaves nodes.
```
   [[ Img 1.1 - JSON-path tree in tuple format ]]

   ----\
       |------> [name] \
       |  {off. cache} |----> [first]      <slot1>
       |               |      {off. cache}
       |               |
       |               |----> [last]       <slot2>
       |                      {off. cache}
       |
       \------> [birthday]   <slot3>
                {off. cache}
```

During tuple creation we will also fill off_cache on intermediate tree nodes.
For example, when first index part "name.first" is built, we would already have name offset and would able to start there parsing only "last" lexeme.
```
s:create_index('name'', { parts = {{ "name.first", "str" },
                                   { "name.last",  "str" }}})
```
The resulting information offset would be stored in slot obtained from tree leaf node.

```
    [[ Img 1.2 - Tuple data stored in database of same format ]]

                                                              | Description  |
   +=======================+======+===================+
   | slot1 | slot2 | slot3 | town |       name        |       | tuple schema |
   +=======+=======+=======+======+============+======+
   |   2   |   3   |   0   | NY   | Ayn | Rand |              |    tuple1    |
   +-----------------------+------+---------+--+------+
   |   2   |   4   |   0   | NY   | Richard | Feynman |       |    tuple2    |
   +-----------------------+------+---------+---------+
                           ^      ^     ^   ^
                           0      2     3   4                 |    offset    |
```

### 1.2. Access data by JSON-path
The central API to access information is a routine
```
static inline const char *
tuple_field_raw(const struct tuple_format *format, const char *tuple,
                const uint32_t *field_map, uint32_t field_no)
```

In most scenarios related to indexes it is called with *field_no* extracted from *key_part* - a descriptor of part representing index.
```
field_b = tuple_field_raw(format_b, tuple_b_raw, field_map_b, part->fieldno);
```
With introducing parts containing JSON path we need to rework such calls.

To avoid parsing JSON path each time we suppose to introduce a cache for *offset_slot* in *key_part* structure:
```
struct key_part {
  /** Tuple field index for this part */
  uint32_t fieldno;
  ...

  /** Data JSON path. */
  const char *data_path;
  /** Epoch of tuple_format cached in offset_slot. */
  uint32_t slot_epoch;
  /** Cache of corresponding tuple_format slot_offset. */
  int32_t slot_cache;
};
```
same as *tuple_format* where such epoch would be initialized with a new bigger(when source key_parts have changed) value on creation.
```
struct tuple_format {
  ...

  /** Epoch of tuple format. */
  uint32_t epoch;
  /** Offset slot tree for JSON indexes. */
  TREE json_tree;
  /** Formats of the fields. */
  struct tuple_field fields[0];
};
```

In all index-related scenarios with *tuple_field_raw* we would rework with a new function:
```
const char *
tuple_field_by_part(const struct tuple *tuple, const struct key_def *def,
                    uint32_t idx);

```

With first argument *tuple* we can obtain tuple_format that contain *epoch* field.
```
  [[ Img 2.1 - Block diagram for tuple_field_by_part routine ]]

            [[def->epoch == tuple_format(tuple)->epoch]]
                                 ||
                  YES            ||             NO
                 ________________/\________________
                |                                  |

     use offset_slot from             PARSE def->parts[idx]->data_path and
  def->parts[idx].cached_slot   observe tuple_format(tuple)->json_tree structure,
                                     UPDATE def->parts[idx].slot_epoch and
                                           def->parts[idx].slot_cache
```

PARSE is about parsing JSON path lexeme-by-lexeme and step down the json_tree until leaf with slot offset would reached.
Then we need to UPDATE key_part cache.


## Rationale and alternatives

### 2.1 JSON-path hash table instead of tree
Instead of JSON-path tree in space format described in chapter (1.1) in previous section we could use hash table in format:
```
   HASTABLE<const char *data_path, int32_t offset_slot>

                            lookup
   "name.first" ----> hash1 ------> [hash1, "name.first", offset_slot1]
   "name.last"  ----> hash2 ------> [hash2, "name.last", offset_slot2]
```
The only drawback of this approach is the inability to speed up the parsing of data on tuple creation based on the information obtained by parsing the previous paths.
An as tuple insertion is rare frequent operation this makes sense.

### 2.2 Re-allocatable slot_cache
Epoch-based invalidation is looking complicated enough.
It is possible to allocate slot_cache[] array for each key_part and store slots for all epoch (where epoch would be an index in that array).
```
                         format->epoch
key_part {                     ||
   slot_epoch[]:               \/
        [1: s1, 2: s2, ... epoch: s3, ... max_epoch: sN]
}

I.e.:
    tuple_map[key_part->slot_epoch[format->epoch]]
```
But as formats are created really often, this approach is really resource-consumable.