Follow up ... got to digging into db's of several moodles yesterday/today seeking something else and noticed, for the first time, a 'adob_logsql' table but only on sites that had been upgraded/marched upwards from 1.9's. There's no data in them and see no evidence of related tables. No such table (s) in fresh 3.x's.
Must be related to external DB for some purpose.
You might be able tell by looking at data contained therein.
adblog_sql table
Field name Type Allow nulls? Key Default value Extras
id bigint(10) unsigned No Primary NULL auto_increment
created datetime No None NULL
sql0 varchar(250) No None
sql1 text Yes None NULL
params text Yes None NULL
tracer text Yes None NULL
timer decimal(16,6) No None 0.000000
'spirit of sharing', Ken