The "innocuous change" was to add an index for "is_robot" which is true
for 6 out of 20,000 records and null for the rest.
My complaint/question/observation is not how to optimize the query
that went awry, but to be alarmed that a venerable and perfectly
serviceable query, written years ago and ignored ever since, suddenly
brought the system crashing down after making a seemingly innocuous
change intended to make a marginal improvement on an unrelated query.
I had previously believed that tinkering the schema by adding indexes was a
safe activity. It's as though I add a shortcut to my regular commute
and caused a massive traffic jam when the entire traffic flow tried to
follow me.
(Both tables are "ok" according to analyze table)
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