Enhance normalized query denormalisation

[rjuju]

For now, pg_qualstats can retrieve the most (executed|filtering...) quals, and use these for denormalize example queries for each kind.

However, denormalisation is sometimes not possible (parameters in the SELECT part or LIMIT for instance). Since it's not really pg_qualstats' job to gather these information, the UI should fallback on a not-normalized version of the query which is gathered by pg_qualstats (pg_qualstats_example_query(queryid)).