[Tarantool-patches] [PATCH 4/4] sql: provide a user friendly frontend for accessing session settings

[Vladislav Shpilevoy]

Hi! Thanks for the fixes!

On the branch you said this commit is 'Part of #4711'. Why
not 'Closes #4711'? What else is left?

>     sql: provide a user friendly frontend for accessing session settings
>     
>     Currently if a user wants to change session setting with sql, he has
>     to execute non-obvious query, thus, we introduce a more native way to
>     do this.
>     
>     Part of #4711
>     
>     @TarantoolBot document
>     Title: API for accessing _session_settings space.
>     There are two ways of updating values of session settings:
>     via Lua and SQL.
>     
>     Lua:
>     box.session.settings is a table, which is accessible right after
>     session creation. The syntax is the following:

Lets say it is available always. Because a user can't live without
a session anyway. It always exists for a user.

>     `box.session.settings.<setting_name>:set(<new_value>)`.
>     
>     Example of usage:
>     ```
>     tarantool> box.session.settings.sql_default_engine
>     ---
>     - memtx
>     ...
>     
>     tarantool> box.session.settings.sql_default_engine:set('vinyl')
>     ---
>     ...
>     
>     ```
>     
>     The table itself represents the (unordered) result of select
>     from _session_settings space. Every setting is implemented as
>     a table, so there is no way to retrieve an actual value and use
>     it until :get() method is introduced.
>     
>     SQL:
>     Instead of typing long UPDATE query one can use the SET statement:
>     `box.execute([[SET "<setting_name>" = <new_value>]])`.
>     Note, that this query is case sensitive so the name must be quoted.
>     
>     Example:
>     ```
>     tarantool> box.execute([[set "sql_default_engine" = 'memtx']])
>     ---
>     - row_count: 1
>     ...
>     
>     tarantool> box.execute([[set "sql_defer_foreign_keys" = true]])
>     ---
>     - row_count: 1
>     ...
>     
>     ```

I am not so sure about binary search anymore. I just found another
probably much faster way - perfect hash function. There is a GNU
tool 'gperf' to generate C code for a given keyset with hash
calculation and search functions.

I fed our settings to it and obtained not so scary result.

File: test.gperf

    %%
    sql_default_engine
    sql_defer_foreign_keys
    sql_full_column_names
    sql_full_metadata
    sql_parser_debug
    sql_recursive_triggers
    sql_reverse_unordered_selects
    sql_select_debug
    sql_vdbe_debug
    %%

Command: gperf test.gperf > test.c

The test.c file is 122 lines long including some {}, useless
checks, macros, and comments. It could easily be compacted a lot.

This is probably overkill though. It needs to be completely
regenerated each time when the keyset is changed, consumes more
static memory.

The patchset LGTM. I propose to send it to Nikita on a second
review.