The problem appeared when I found the queries suddenly taking longer
than usual. Investigation showed that execution time greatly depends
on the way the queries are invoked.
Consider fn(x) simply a macro containing a plain SQL SELECT statement
returning SETOF (further detail follows below):

# SELECT fn(x);
-> 6.3 sec.

# SELECT a from fn(x) as a;
-> 1.3 sec.

Further investigation with auto_explain shows different plans being
chosen. The slower one uses an Index Only Scan, which seems to perform
bad. Slightly increasing random_page_cost solves this, but this seems
the wrong way, because we are on SSD+ZFS, where random_page_cost
actually should be DEcreased, as there is no difference if random or

During this effort I accidentally came upon from_collapse_limit,
and setting it off significantly changed things:

# SET from_collapse_limit = 1;

# SELECT fn(x);
-> 0.6 sec.

# SELECT a from fn(x) as a;
-> 1.2 sec.

The plans look different now (obviousely), and again the difference
between the two invocations comes from an an Index Only Scan, but
this time the Index Only Scan is faster. So now we can reduce
random_page_cost in order to better reflect physical circumstances,
and then both invocations will be fast.

>From here it looks like from_collapse is the problem.

Now for the details:

VACUUM ANALYZE is up to date, and all respective configurations are as

The query itself contains three nested SELECTS working all on the same
table. The table is 400'000 rows, 36 MB. (The machine is a pentium-3,
which is my router - so don't be surprized about the comparatively long
execution times.)

This is the (critical part of the) query - let $1 be something like

  SELECT MAX(quotes.datum) AS ratedate, aktkurs.*
    FROM quotes, wpnames, places,
     (SELECT quotes.datum, close, quotes.wpname_id, places.waehrung
        FROM quotes, wpnames, places,
         (SELECT MAX(datum) AS datum, wpname_id
            FROM quotes
            WHERE datum <= $1
            GROUP BY wpname_id) AS newest
        WHERE newest.datum = quotes.datum
          AND newest.wpname_id = quotes.wpname_id
          AND quotes.wpname_id =
          AND wpnames.place_id = AS aktkurs
    WHERE quotes.wpname_id =
      AND wpnames.place_id = AND places.platz = 'WAEHR'
      AND wpnames.nummer = aktkurs.waehrung
      AND quotes.datum <= aktkurs.datum
    GROUP BY aktkurs.datum, aktkurs.close, aktkurs.wpname_id,

Here are the (respective parts of the) tables:

CREATE TABLE public.quotes -- rows = 405466, 36 MB
  id integer NOT NULL DEFAULT nextval('quotes_id_seq'::regclass),
  wpname_id integer NOT NULL,
  datum date NOT NULL,
  close double precision NOT NULL,
  CONSTRAINT quotes_pkey PRIMARY KEY (id),
  CONSTRAINT fk_rails_626c320689 FOREIGN KEY (wpname_id)
      REFERENCES public.wpnames (id) MATCH SIMPLE
CREATE INDEX quotes_wd_idx -- 8912 kB
  ON public.quotes
  USING btree
  (wpname_id, datum);

CREATE TABLE public.wpnames -- rows = 357, 40 kB
  id integer NOT NULL DEFAULT nextval('wpnames_id_seq'::regclass),
  place_id integer NOT NULL,
  nummer text NOT NULL,
  name text NOT NULL,
  CONSTRAINT wpnames_pkey PRIMARY KEY (id),
  CONSTRAINT fk_rails_18eae07552 FOREIGN KEY (place_id)
      REFERENCES public.places (id) MATCH SIMPLE
CREATE TABLE public.places -- rows = 11, 8192 b
  id integer NOT NULL DEFAULT nextval('places_id_seq'::regclass),
  platz text NOT NULL,
  text text,
  waehrung character varying(3) NOT NULL,
  CONSTRAINT places_pkey PRIMARY KEY (id)

Hint: the quotes table contains daily stock quotes AND forex quotes,
and what the thing does is fetch the newest quotes before a given
date (inmost SELECT), fetch the respective currency ("waehrung") from
wpnames+places (next SELECT), and fetch the (date of the) respective
newest forex quote (last SELECT). (A final outermost fourth select
will then put it all together, but thats not part of the problem.)

Finally, the execution plans:

6 sec. index only scan with from_collapse:

1.3 sec. seq scan with from_collapse:

1.2 sec. seq scan w/o from_collapse:

0.6 sec. index only scan w/o from_collapse:

Addendum: from the Guides for the mailing list, supplemental
information as requested. As this concerns planner strategy, which is
influenced by statistics, it appears difficult to me to create a
proper test-case, because I would need to know from where the planner
fetches the decision-relevant information - which is exactly my
question: how does it get the clue to choose the bad plans?

 CPU: Intel Pentium III (945.02-MHz 686-class CPU)
 avail memory = 2089263104 (1992 MB)
 FreeBSD 11.1-RELEASE-p7
 PostgreSQL 9.5.7 on i386-portbld-freebsd11.1, compiled by FreeBSD clang 
version 4.0.0 (tags/RELEASE_400/final 297347) (based on LLVM 4.0.0), 32-bit

             name             |            current_setting             |       
 application_name             | psql                                   | client
 archive_command              | ~pgsql/autojobs/RedoLog.copy "%f" "%p" | 
configuration file
 archive_mode                 | on                                     | 
configuration file
 autovacuum                   | off                                    | 
configuration file
 autovacuum_naptime           | 5min                                   | 
configuration file
 checkpoint_completion_target | 0                                      | 
configuration file
 checkpoint_timeout           | 10min                                  | 
configuration file
 client_encoding              | UTF8                                   | client
 DateStyle                    | German, DMY                            | 
configuration file
 default_text_search_config   | pg_catalog.german                      | 
configuration file
 dynamic_shared_memory_type   | posix                                  | 
configuration file
 effective_cache_size         | 1GB                                    | 
configuration file
 effective_io_concurrency     | 2                                      | 
configuration file
 full_page_writes             | off                                    | 
configuration file
 lc_messages                  | en_US.UTF-8                            | 
configuration file
 lc_monetary                  | en_US.UTF-8                            | 
configuration file
 lc_numeric                   | en_US.UTF-8                            | 
configuration file
 lc_time                      | de_DE.UTF-8                            | 
configuration file
 listen_addresses             |,             | 
configuration file
 log_checkpoints              | on                                     | 
configuration file
 log_connections              | on                                     | 
configuration file
 log_destination              | syslog                                 | 
configuration file
 log_disconnections           | on                                     | 
configuration file
 log_error_verbosity          | terse                                  | 
configuration file
 log_line_prefix              | %u:%d[%r]                              | 
configuration file
 log_lock_waits               | on                                     | 
configuration file
 log_min_duration_statement   | 1min                                   | 
configuration file
 log_min_messages             | info                                   | 
configuration file
 log_temp_files               | 10000kB                                | 
configuration file
 maintenance_work_mem         | 350MB                                  | 
configuration file
 max_connections              | 60                                     | 
configuration file
 max_files_per_process        | 200                                    | 
configuration file
 max_stack_depth              | 60MB                                   | 
configuration file
 max_wal_size                 | 1GB                                    | 
configuration file
 min_wal_size                 | 80MB                                   | 
configuration file
 shared_buffers               | 180MB                                  | 
configuration file
 synchronous_commit           | on                                     | 
configuration file
 temp_buffers                 | 80MB                                   | 
configuration file
 unix_socket_permissions      | 0777                                   | 
configuration file
 wal_buffers                  | 256kB                                  | 
configuration file
 wal_level                    | archive                                | 
configuration file
 wal_writer_delay             | 2s                                     | 
configuration file
 work_mem                     | 350MB                                  | 
configuration file

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