Explain not showing memory usage - postgresql

Question

I am trying to see how much memory a query takes. In articles I can see in their explain output has a memory value but when I run explain I do not get a memory value.
Here is my query:
explain (analyze, verbose, buffers) select * from watched_url_queue
join rate_check rc on watched_url_queue.domain_key = rc.domain_key
where rc.locked = false
order by rc.domain_key
limit 1;
And this is my output:
Limit (cost=0.70..0.85 rows=1 width=336) (actual time=0.009..0.011 rows=1 loops=1)
Output: watched_url_queue.watched_url_record_id, watched_url_queue.url, watched_url_queue.domain_key, watched_url_queue.targets, watched_url_queue.create_date, watched_url_queue.tries, watched_url_queue.defer_until, watched_url_queue.duration, watched_url_queue.user_auth_custom_id, watched_url_queue.completed, rc.domain_key, rc.last_scan, rc.locked, rc.domain_key
Buffers: shared hit=7
-> Merge Join (cost=0.70..32514.53 rows=219864 width=336) (actual time=0.009..0.009 rows=1 loops=1)
Output: watched_url_queue.watched_url_record_id, watched_url_queue.url, watched_url_queue.domain_key, watched_url_queue.targets, watched_url_queue.create_date, watched_url_queue.tries, watched_url_queue.defer_until, watched_url_queue.duration, watched_url_queue.user_auth_custom_id, watched_url_queue.completed, rc.domain_key, rc.last_scan, rc.locked, rc.domain_key
Inner Unique: true
Merge Cond: (watched_url_queue.domain_key = rc.domain_key)
Buffers: shared hit=7
-> Index Scan using idx_watchedurlqueue_domainkey on public.watched_url_queue (cost=0.42..29069.88 rows=439728 width=289) (actual time=0.003..0.003 rows=1 loops=1)
Output: watched_url_queue.watched_url_record_id, watched_url_queue.url, watched_url_queue.domain_key, watched_url_queue.targets, watched_url_queue.create_date, watched_url_queue.tries, watched_url_queue.defer_until, watched_url_queue.duration, watched_url_queue.user_auth_custom_id, watched_url_queue.completed
Buffers: shared hit=4
-> Index Scan using rate_check_pkey on public.rate_check rc (cost=0.28..141.85 rows=2519 width=28) (actual time=0.003..0.003 rows=1 loops=1)
Output: rc.domain_key, rc.last_scan, rc.locked
Filter: (NOT rc.locked)
Buffers: shared hit=3
Planning time: 0.362 ms
Execution time: 0.060 ms
How do I see the memory usage?

Answer 1

A Merge Join doesn't report memory usage because it doesn't need any buffers (like e.g. the hash join). It simply goes through the sorted data from the two indexes as they are retrieved without the need to buffer that.
If you want to estimate the overall size of each step in the plan, you can multiply the width value and the rows value (from the actual rows).
But that is not necessarily the "memory" that is needed by the query, as the blocks are managed in the shared memory ("cache") rather then "by the query".