It’s like writing a compiler or interpreter: writing one is easy; writing a good one extremely hard.
This suggester isn’t very good. It takes a single query and suggests indexes for it. A good one would take a mix of queries and suggest a set of indexes, also considering the impact on write speed of additional indexes (table updates often need to update indexes, too)
For the example in this article, if the table is large and the average number of rows with a given ‘a’ value is close to 1 or if most queries are for ‘a’ values that aren’t in the database, it may even be better to do
CREATE INDEX x1a ON x1(a);