Some time ago I was contacted by Adam Smith – he pointed out that subquery names in “Subquery Scan" nodes were not properly anonymized.
Now, they are, which you can see in here:
While working on it, I also added (helpful?) links from node types to my blogposts about reading explain output – Explaining the unexplainable.
In previous posts in this series, I talked about how to read EXPLAIN output, and what each line (operation/node) means.
Now, in the final post, I will try to explain how it happens that Pg chooses “Operation X" over “Operation Y".
In this, hopefully 2nd to last, post in the series, I will cover the rest of usually happening operations that you can see in your explain outputs.
In previous post in the series I wrote about how to interpret single line in explain analyze output, it's structure, and later on described all basic data-getting operations (nodes in explain tree).
Today, we'll move towards more complicated operations.
Last time I wrote about what explain output shows. Now I'd like to talk more about various types of “nodes" / operations that you might see in explain plans.
One of the first things new DBA hears is “Use the EXPLAIN". And upon first try he/she is greeted with incomprehensible:
QUERY PLAN --------------------------------------------------------------------------------------------------------------------------- Sort (cost=146.63..148.65 ROWS=808 width=138) (actual TIME=55.009..55.012 ROWS=71 loops=1) Sort KEY: n.nspname, p.proname, (pg_get_function_arguments(p.oid)) Sort Method: quicksort Memory: 43kB -> Hash JOIN (cost=1.14..107.61 ROWS=808 width=138) (actual TIME=42.495..54.854 ROWS=71 loops=1) Hash Cond: (p.pronamespace = n.oid) -> Seq Scan ON pg_proc p (cost=0.00..89.30 ROWS=808 width=78) (actual TIME=0.052..53.465 ROWS=2402 loops=1) FILTER: pg_function_is_visible(oid) -> Hash (cost=1.09..1.09 ROWS=4 width=68) (actual TIME=0.011..0.011 ROWS=4 loops=1) Buckets: 1024 Batches: 1 Memory Usage: 1kB -> Seq Scan ON pg_namespace n (cost=0.00..1.09 ROWS=4 width=68) (actual TIME=0.005..0.007 ROWS=4 loops=1) FILTER: ((nspname <> 'pg_catalog'::name) AND (nspname <> 'information_schema'::name))
What does it even mean?