Considerations:
- It is much harder to produce EXPLAIN FORMAT=JSON output than tabular format output.
- IIRC, MySQL supports EXPLAIN FORMAT=JSON FOR CONNECTION, but the output lacks some details
- Because of the above, we can start by supporting a tabular form in EXPLAIN FOR CONNECTION.
- Can we print ANALYZE [ FORMAT=JSON] for the other thread?
Review input: https://lists.launchpad.net/maria-developers/msg13074.html
Yes, I'm surprised how this did work on my machine. I added the initialization and force-pushed into the same branch.
Ok. Please in the future do not have two branches with similar names like bb-10.8-MDEV-10000 and bb-10.8-mdev-10000. This is confusing 
One other thing to consider:
In MySQL:
EXPLAIN FOR CONNECTION produces "partitions" and "filtered" columns.
These columns are also part of regular EXPLAIN output (Initially
they weren't and one had to use EXPLAIN EXTENDED to get the "filtered"
column and EXPLAIN PARTITIONS to get the "partitions")
In MariaDB:
EXPLAIN doesn't produce "partitions" or "filtered". One needs to use
EXPLAIN (EXTENDED|PARTITIONS).
The current patch doesn't produce "partitions" or "filtered".
But since we're aiming at MySQL compatibility perhaps we should do that?
Sorry for the confusion, bb-10.8-MDEV-10000 is the actual branch. I removed the other one having lowercase mdev.
Regarding "partitions" and "filtered": is we add these fields only to EXPLAIN FOR CONNECTION output wouldn't it be inconsistent that SHOW EXPLAIN displays one set of columns and EXPLAIN FOR CONNECTION - another one? Doesn't it make sense to add those fields to SHOW EXPLAIN output as well?
Filed MDEV-27776 about addressing filtered and partitions column.