box.space._session_settings

A temporary system space with settings that affect behavior, particularly SQL behavior, for the current session. It uses a special engine named 'service'. Every 'service' tuple is created on the fly, that is, new tuples are made every time _session_settings is accessed. Every settings tuple has two fields: name (the primary key) and value. The tuples' names and default values are:

• sql_default_engine: default storage engine for new SQL tables. Default: memtx.
• sql_full_column_names: use full column names in SQL result set metadata. Default: false.
• sql_full_metadata: whether SQL result set metadata includes more than just name and type. Default:false.
• sql_parser_debug: show parser steps for following statements. Default: false.
• sql_recursive_triggers: whether a triggered statement can activate a trigger. Default: true.
• sql_reverse_unordered_selects: return result rows in reverse order if there is no ORDER BY clause. Default: false.
• sql_select_debug: show execution steps during SELECT. Default:false.
• sql_seq_scan: allow sequential scans in SQL SELECT. Default: true.
• sql_vdbe_debug: for internal use. Default:false.
• sql_defer_foreign_keys (removed in 2.11.0): whether foreign-key checks can wait till commit. Default: false.
• error_marshaling_enabled (removed in 2.10.0): whether error objects have a special structure. Default: false.

Three requests are possible: select, get and update. For example, after s = box.space._session_settings, s:select('sql_default_engine') probably returns {'sql_default_engine', 'memtx'}, and s:update('sql_default_engine', {{'=', 'value', 'vinyl'}}) changes the default engine to 'vinyl'.
Updating sql_parser_debug or sql_select_debug or sql_vdbe_debug has no effect unless Tarantool was built with -DCMAKE_BUILD_TYPE=Debug. To check if this is so, look at require('tarantool').build.target.