Unexpected SQL queries to Postgres database on Rails/Heroku

Question

I was diving into a really long request to one of my Rails applications using NewRelic and found a number of SQL queries that appear entirely foreign that are taking up a significant length of time. I've Google'd around but I've come up empty handed as to what they are, let alone whether I can prevent them from occurring.

SELECT COUNT(*) FROM pg_class c LEFT JOIN pg_namespace n ON n.oid = c.relnamespace WHERE c.relkind in (?, ?) AND c.relname = ? AND n.nspname = ANY (current_schemas(false))

…and…

SELECT a.attname, format_type(a.atttypid, a.atttypmod), pg_get_expr(d.adbin, d.adrelid), a.attnotnull, a.atttypid, a.atttypmod FROM pg_attribute a LEFT JOIN pg_attrdef d ON a.attrelid = d.adrelid AND a.attnum = d.adnum WHERE a.attrelid = ?::regclass AND a.attnum > ? AND NOT a.attisdropped ORDER BY a.attnum

…each occurred 7 times, taking 145ms and 135ms (respectively) total.

SELECT DISTINCT(attr.attname) FROM pg_attribute attr INNER JOIN pg_depend dep ON attr.attrelid = dep.refobjid AND attr.attnum = dep.refobjsubid INNER JOIN pg_constraint cons ON attr.attrelid = cons.conrelid AND attr.attnum = cons.conkey[?] WHERE cons.contype = ? AND dep.refobjid = ?::regclass

…was performed 2 times at a cost of 104ms, and…

SHOW search_path

…commanded 45ms in a single call.

My gut says these are related to the Postgres Rails adapter, but I don't understand what triggers them or what they're doing, or (more importantly) why they fired during a typical request.

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I just checked out the logs more thoroughly and it looks like the Dyno this request ran on had been transitioned to "up" just a few seconds earlier, so it's likely this request was the first.

Answer 1

The tables pg_class, pg_attribute, pg_depend etc all describe table, columns and dependencies in postgres. In Rails, model classes are defined by the tables, so Rails reads the tables and columns to figure out the attributes for each model.

In development mode it looks up these values everytime the model is accessed, so if you've mad e a recent change, Rails knows about it. In production mode, Rails caches this so you would see these much less frequently, and so it really isn't a concern.