Tag: unexplainable

Waiting for PostgreSQL 19 – Display Memoize planner estimates in EXPLAIN

On 29th of July 2025, David Rowley committed patch:

Display Memoize planner estimates in EXPLAIN
 
There've been a few complaints that it can be overly difficult to figure
out why the planner picked a Memoize plan.  To help address that, here we
adjust the EXPLAIN output to display the following additional details:
 
1) The estimated number of cache entries that can be stored at once
2) The estimated number of unique lookup keys that we expect to see
3) The number of lookups we expect
4) The estimated hit ratio
 
Technically #4 can be calculated using #1, #2 and #3, but it's not a
particularly obvious calculation, so we opt to display it explicitly.
The original patch by Lukas Fittl only displayed the hit ratio, but
there was a fear that might lead to more questions about how that was
calculated.  The idea with displaying all 4 is to be transparent which
may allow queries to be tuned more easily.  For example, if #2 isn't
correct then maybe extended statistics or a manual n_distinct estimate can
be used to help fix poor plan choices.
 
Author: Ilia Evdokimov <[email protected]>
Author: Lukas Fittl <[email protected]>
Reviewed-by: David Rowley <[email protected]>
Reviewed-by: Andrei Lepikhov <[email protected]>
Reviewed-by: Robert Haas <[email protected]>
Discussion: https://postgr.es/m/CAP53Pky29GWAVVk3oBgKBDqhND0BRBN6yTPeguV_qSivFL5N_g%40mail.gmail.com

Continue reading Waiting for PostgreSQL 19 – Display Memoize planner estimates in EXPLAIN

explain.depesz.com changes and new stats

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.

Continue reading explain.depesz.com changes and new stats

Explaining the unexplainable – part 5

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".

Continue reading Explaining the unexplainable – part 5

Explaining the unexplainable – part 3

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.

Continue reading Explaining the unexplainable – part 3

Explaining the unexplainable

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?

Continue reading Explaining the unexplainable