BUG #15032: Segmentation fault when running a particular query

The following bug has been logged on the website:

Bug reference:      15032
Logged by:          Guo Xiang Tan
Email address:      gxtan1990@gmail.com
PostgreSQL version: 10.1
Operating system:   Ubuntu 14.04.4 LTS
Description:

## `SELECT version()`

`PostgreSQL 10.1 on x86_64-pc-linux-gnu, compiled by gcc (Ubuntu
5.4.0-6ubuntu1~16.04.4) 5.4.0 20160609, 64-bit`

## Query that results in segmentation fault

```
UPDATE posts
SET avg_time = (x.gmean / 1000)
FROM (SELECT post_timings.topic_id,
             post_timings.post_number,
             round(exp(avg(CASE WHEN msecs > 0 THEN ln(msecs) ELSE 0 END)))
AS gmean
      FROM post_timings
      INNER JOIN posts AS p2
        ON p2.post_number = post_timings.post_number
          AND p2.topic_id = post_timings.topic_id
          AND p2.user_id <> post_timings.user_id
      GROUP BY post_timings.topic_id, post_timings.post_number) AS x
WHERE (x.topic_id = posts.topic_id
  AND x.post_number = posts.post_number
  AND (posts.avg_time <> (x.gmean / 1000)::int OR posts.avg_time IS
NULL));
```

## Error in PostgreSQL log file

```
[163]: [15-1] LOG:  server process (PID 1983) was terminated by signal 11:
Segmentation fault
[163]: [16-1] DETAIL:  Failed process was running: UPDATE posts
    SET avg_time = (x.gmean / 1000)
    FROM (SELECT post_timings.topic_id,
                 post_timings.post_number,
                 round(exp(avg(CASE WHEN msecs > 0 THEN ln(msecs) ELSE 0 END)))
AS gmean
          FROM post_timings
          INNER JOIN posts AS p2
            ON p2.post_number = post_timings.post_number
              AND p2.topic_id = post_timings.topic_id
              AND p2.user_id <> post_timings.user_id
          GROUP BY post_timings.topic_id, post_timings.post_number) AS x
    WHERE (x.topic_id = posts.topic_id
      AND x.post_number = posts.post_number
      AND (posts.avg_time <> (x.gmean / 1000)::int OR posts.avg_time IS
NULL))
    ;
[163]: [17-1] LOG:  terminating any other active server processes
[2003]: [4-1] WARNING:  terminating connection because of crash of another
server process
[2003]: [5-1] DETAIL:  The postmaster has commanded this server process to
roll back the current transaction and exit, because another server process
exited abnormally and possibly corrupted shared memory.
[2003]: [6-1] HINT:  In a moment you should be able to reconnect to the
database and repeat your command.
[2015]: [4-1] WARNING:  terminating connection because of crash of another
server process
[2015]: [5-1] DETAIL:  The postmaster has commanded this server process to
roll back the current transaction and exit, because another server process
exited abnormally and possibly corrupted shared memory.
```

## pg_dump for relevant tables


```
--
-- PostgreSQL database dump
--

-- Dumped from database version 10.1
-- Dumped by pg_dump version 10.1

SET statement_timeout = 0;
SET lock_timeout = 0;
SET idle_in_transaction_session_timeout = 0;
SET client_encoding = 'UTF8';
SET standard_conforming_strings = on;
SET check_function_bodies = false;
SET client_min_messages = warning;
SET row_security = off;

SET search_path = public, pg_catalog;

SET default_tablespace = '';

SET default_with_oids = false;

--
-- Name: posts; Type: TABLE; Schema: public; Owner: -
--

CREATE TABLE posts (
    id integer NOT NULL,
    user_id integer,
    topic_id integer NOT NULL,
    post_number integer NOT NULL,
    raw text NOT NULL,
    cooked text NOT NULL,
    created_at timestamp without time zone NOT NULL,
    updated_at timestamp without time zone NOT NULL,
    reply_to_post_number integer,
    reply_count integer DEFAULT 0 NOT NULL,
    quote_count integer DEFAULT 0 NOT NULL,
    deleted_at timestamp without time zone,
    off_topic_count integer DEFAULT 0 NOT NULL,
    like_count integer DEFAULT 0 NOT NULL,
    incoming_link_count integer DEFAULT 0 NOT NULL,
    bookmark_count integer DEFAULT 0 NOT NULL,
    avg_time integer,
    score double precision,
    reads integer DEFAULT 0 NOT NULL,
    post_type integer DEFAULT 1 NOT NULL,
    vote_count integer DEFAULT 0 NOT NULL,
    sort_order integer,
    last_editor_id integer,
    hidden boolean DEFAULT false NOT NULL,
    hidden_reason_id integer,
    notify_moderators_count integer DEFAULT 0 NOT NULL,
    spam_count integer DEFAULT 0 NOT NULL,
    illegal_count integer DEFAULT 0 NOT NULL,
    inappropriate_count integer DEFAULT 0 NOT NULL,
    last_version_at timestamp without time zone NOT NULL,
    user_deleted boolean DEFAULT false NOT NULL,
    reply_to_user_id integer,
    percent_rank double precision DEFAULT 1.0,
    notify_user_count integer DEFAULT 0 NOT NULL,
    like_score integer DEFAULT 0 NOT NULL,
    deleted_by_id integer,
    edit_reason character varying(255),
    word_count integer,
    version integer DEFAULT 1 NOT NULL,
    cook_method integer DEFAULT 1 NOT NULL,
    wiki boolean DEFAULT false NOT NULL,
    baked_at timestamp without time zone,
    baked_version integer,
    hidden_at timestamp without time zone,
    self_edits integer DEFAULT 0 NOT NULL,
    reply_quoted boolean DEFAULT false NOT NULL,
    via_email boolean DEFAULT false NOT NULL,
    raw_email text,
    public_version integer DEFAULT 1 NOT NULL,
    action_code character varying(255),
    image_url character varying
);


--
-- Name: TABLE posts; Type: COMMENT; Schema: public; Owner: -
--

COMMENT ON TABLE posts IS 'If you want to query public posts only, use the
badge_posts view.';


--
-- Name: COLUMN posts.post_number; Type: COMMENT; Schema: public; Owner: -
--

COMMENT ON COLUMN posts.post_number IS 'The position of this post in the
topic. The pair (topic_id, post_number) forms a natural key on the posts
table.';


--
-- Name: COLUMN posts.raw; Type: COMMENT; Schema: public; Owner: -
--

COMMENT ON COLUMN posts.raw IS 'The raw Markdown that the user entered into
the composer.';


--
-- Name: COLUMN posts.cooked; Type: COMMENT; Schema: public; Owner: -
--

COMMENT ON COLUMN posts.cooked IS 'The processed HTML that is presented in a
topic.';


--
-- Name: COLUMN posts.reply_to_post_number; Type: COMMENT; Schema: public;
Owner: -
--

COMMENT ON COLUMN posts.reply_to_post_number IS 'If this post is a reply to
another, this column is the post_number of the post it''s replying to. [FKEY
posts.topic_id, posts.post_number]';


--
-- Name: COLUMN posts.reply_quoted; Type: COMMENT; Schema: public; Owner:
-
--

COMMENT ON COLUMN posts.reply_quoted IS 'This column is true if the post
contains a quote-reply, which causes the in-reply-to indicator to be
absent.';


--
-- Name: topics; Type: TABLE; Schema: public; Owner: -
--

CREATE TABLE topics (
    id integer NOT NULL,
    title character varying(255) NOT NULL,
    last_posted_at timestamp without time zone,
    created_at timestamp without time zone NOT NULL,
    updated_at timestamp without time zone NOT NULL,
    views integer DEFAULT 0 NOT NULL,
    posts_count integer DEFAULT 0 NOT NULL,
    user_id integer,
    last_post_user_id integer NOT NULL,
    reply_count integer DEFAULT 0 NOT NULL,
    featured_user1_id integer,
    featured_user2_id integer,
    featured_user3_id integer,
    avg_time integer,
    deleted_at timestamp without time zone,
    highest_post_number integer DEFAULT 0 NOT NULL,
    image_url character varying(255),
    like_count integer DEFAULT 0 NOT NULL,
    incoming_link_count integer DEFAULT 0 NOT NULL,
    category_id integer,
    visible boolean DEFAULT true NOT NULL,
    moderator_posts_count integer DEFAULT 0 NOT NULL,
    closed boolean DEFAULT false NOT NULL,
    archived boolean DEFAULT false NOT NULL,
    bumped_at timestamp without time zone NOT NULL,
    has_summary boolean DEFAULT false NOT NULL,
    vote_count integer DEFAULT 0 NOT NULL,
    archetype character varying(255) DEFAULT 'regular'::character varying
NOT NULL,
    featured_user4_id integer,
    notify_moderators_count integer DEFAULT 0 NOT NULL,
    spam_count integer DEFAULT 0 NOT NULL,
    pinned_at timestamp without time zone,
    score double precision,
    percent_rank double precision DEFAULT 1.0 NOT NULL,
    subtype character varying(255),
    slug character varying(255),
    deleted_by_id integer,
    participant_count integer DEFAULT 1,
    word_count integer,
    excerpt character varying(1000),
    pinned_globally boolean DEFAULT false NOT NULL,
    pinned_until timestamp without time zone,
    fancy_title character varying(400),
    highest_staff_post_number integer DEFAULT 0 NOT NULL,
    featured_link character varying,
    CONSTRAINT has_category_id CHECK (((category_id IS NOT NULL) OR
((archetype)::text <> 'regular'::text))),
    CONSTRAINT pm_has_no_category CHECK (((category_id IS NULL) OR
((archetype)::text <> 'private_message'::text)))
);


--
-- Name: TABLE topics; Type: COMMENT; Schema: public; Owner: -
--

COMMENT ON TABLE topics IS 'To query public topics only: SELECT ... FROM
topics t LEFT INNER JOIN categories c ON (t.category_id = c.id AND
c.read_restricted = false)';


--
-- Name: post_timings; Type: TABLE; Schema: public; Owner: -
--

CREATE TABLE post_timings (
    topic_id integer NOT NULL,
    post_number integer NOT NULL,
    user_id integer NOT NULL,
    msecs integer NOT NULL
);


--
-- Name: posts_id_seq; Type: SEQUENCE; Schema: public; Owner: -
--

CREATE SEQUENCE posts_id_seq
    START WITH 1
    INCREMENT BY 1
    NO MINVALUE
    NO MAXVALUE
    CACHE 1;


--
-- Name: posts_id_seq; Type: SEQUENCE OWNED BY; Schema: public; Owner: -
--

ALTER SEQUENCE posts_id_seq OWNED BY posts.id;


--
-- Name: topics_id_seq; Type: SEQUENCE; Schema: public; Owner: -
--

CREATE SEQUENCE topics_id_seq
    START WITH 1
    INCREMENT BY 1
    NO MINVALUE
    NO MAXVALUE
    CACHE 1;


--
-- Name: topics_id_seq; Type: SEQUENCE OWNED BY; Schema: public; Owner: -
--

ALTER SEQUENCE topics_id_seq OWNED BY topics.id;


--
-- Name: posts id; Type: DEFAULT; Schema: public; Owner: -
--

ALTER TABLE ONLY posts ALTER COLUMN id SET DEFAULT
nextval('posts_id_seq'::regclass);


--
-- Name: topics id; Type: DEFAULT; Schema: public; Owner: -
--

ALTER TABLE ONLY topics ALTER COLUMN id SET DEFAULT
nextval('topics_id_seq'::regclass);


--
-- Name: topics forum_threads_pkey; Type: CONSTRAINT; Schema: public; Owner:
-
--

ALTER TABLE ONLY topics
    ADD CONSTRAINT forum_threads_pkey PRIMARY KEY (id);


--
-- Name: posts posts_pkey; Type: CONSTRAINT; Schema: public; Owner: -
--

ALTER TABLE ONLY posts
    ADD CONSTRAINT posts_pkey PRIMARY KEY (id);


--
-- Name: idx_posts_created_at_topic_id; Type: INDEX; Schema: public; Owner:
-
--

CREATE INDEX idx_posts_created_at_topic_id ON posts USING btree (created_at,
topic_id) WHERE (deleted_at IS NULL);


--
-- Name: idx_posts_deleted_posts; Type: INDEX; Schema: public; Owner: -
--

CREATE INDEX idx_posts_deleted_posts ON posts USING btree (topic_id,
post_number) WHERE (deleted_at IS NOT NULL);


--
-- Name: idx_posts_user_id_deleted_at; Type: INDEX; Schema: public; Owner:
-
--

CREATE INDEX idx_posts_user_id_deleted_at ON posts USING btree (user_id)
WHERE (deleted_at IS NULL);


--
-- Name: idx_topics_front_page; Type: INDEX; Schema: public; Owner: -
--

CREATE INDEX idx_topics_front_page ON topics USING btree (deleted_at,
visible, archetype, category_id, id);


--
-- Name: idx_topics_user_id_deleted_at; Type: INDEX; Schema: public; Owner:
-
--

CREATE INDEX idx_topics_user_id_deleted_at ON topics USING btree (user_id)
WHERE (deleted_at IS NULL);


--
-- Name: idxtopicslug; Type: INDEX; Schema: public; Owner: -
--

CREATE INDEX idxtopicslug ON topics USING btree (slug) WHERE ((deleted_at IS
NULL) AND (slug IS NOT NULL));


--
-- Name: index_forum_threads_on_bumped_at; Type: INDEX; Schema: public;
Owner: -
--

CREATE INDEX index_forum_threads_on_bumped_at ON topics USING btree
(bumped_at DESC);


--
-- Name: index_post_timings_on_user_id; Type: INDEX; Schema: public; Owner:
-
--

CREATE INDEX index_post_timings_on_user_id ON post_timings USING btree
(user_id);


--
-- Name: index_posts_on_reply_to_post_number; Type: INDEX; Schema: public;
Owner: -
--

CREATE INDEX index_posts_on_reply_to_post_number ON posts USING btree
(reply_to_post_number);


--
-- Name: index_posts_on_topic_id_and_post_number; Type: INDEX; Schema:
public; Owner: -
--

CREATE UNIQUE INDEX index_posts_on_topic_id_and_post_number ON posts USING
btree (topic_id, post_number);


--
-- Name: index_posts_on_user_id_and_created_at; Type: INDEX; Schema: public;
Owner: -
--

CREATE INDEX index_posts_on_user_id_and_created_at ON posts USING btree
(user_id, created_at);


--
-- Name: index_topics_on_created_at_and_visible; Type: INDEX; Schema:
public; Owner: -
--

CREATE INDEX index_topics_on_created_at_and_visible ON topics USING btree
(created_at, visible) WHERE ((deleted_at IS NULL) AND ((archetype)::text <>
'private_message'::text));


--
-- Name: index_topics_on_id_and_deleted_at; Type: INDEX; Schema: public;
Owner: -
--

CREATE INDEX index_topics_on_id_and_deleted_at ON topics USING btree (id,
deleted_at);


--
-- Name: index_topics_on_lower_title; Type: INDEX; Schema: public; Owner:
-
--

CREATE INDEX index_topics_on_lower_title ON topics USING btree
(lower((title)::text));


--
-- Name: index_topics_on_pinned_at; Type: INDEX; Schema: public; Owner: -
--

CREATE INDEX index_topics_on_pinned_at ON topics USING btree (pinned_at)
WHERE (pinned_at IS NOT NULL);


--
-- Name: index_topics_on_pinned_globally; Type: INDEX; Schema: public;
Owner: -
--

CREATE INDEX index_topics_on_pinned_globally ON topics USING btree
(pinned_globally) WHERE pinned_globally;


--
-- Name: post_timings_summary; Type: INDEX; Schema: public; Owner: -
--

CREATE INDEX post_timings_summary ON post_timings USING btree (topic_id,
post_number);


--
-- Name: post_timings_unique; Type: INDEX; Schema: public; Owner: -
--

CREATE UNIQUE INDEX post_timings_unique ON post_timings USING btree
(topic_id, post_number, user_id);


--
-- PostgreSQL database dump complete
--
```

## Stack trace after following
https://wiki.edubuntu.org/DebuggingProgramCrash

```
Program received signal SIGSEGV, Segmentation fault.
index_markpos (scan=0x0) at
/build/postgresql-10-qAeTPy/postgresql-10-10.1/build/../src/backend/access/index/indexam.c:373
373    /build/postgresql-10-qAeTPy/postgresql-10-10.1/build/../src/backend/access/index/indexam.c:
No such file or directory.

(gdb) bt
#0  index_markpos (scan=0x0) at
/build/postgresql-10-qAeTPy/postgresql-10-10.1/build/../src/backend/access/index/indexam.c:373
#1  0x000055a812746c68 in ExecMergeJoin (pstate=0x55a8131bc778) at
/build/postgresql-10-qAeTPy/postgresql-10-10.1/build/../src/backend/executor/nodeMergejoin.c:1188
#2  0x000055a81272cf3f in ExecProcNode (node=0x55a8131bc778) at
/build/postgresql-10-qAeTPy/postgresql-10-10.1/build/../src/include/executor/executor.h:250
#3  EvalPlanQualNext (epqstate=epqstate@entry=0x55a81318c518) at
/build/postgresql-10-qAeTPy/postgresql-10-10.1/build/../src/backend/executor/execMain.c:3005
#4  0x000055a81272d342 in EvalPlanQual (estate=estate@entry=0x55a81318c018,
epqstate=epqstate@entry=0x55a81318c518,
relation=relation@entry=0x7f4e4e25ab68, rti=1, lockmode=<optimized out>,
tid=tid@entry=0x7ffd54492330, priorXmax=8959603) at
/build/postgresql-10-qAeTPy/postgresql-10-10.1/build/../src/backend/executor/execMain.c:2521
#5  0x000055a812747af7 in ExecUpdate (mtstate=mtstate@entry=0x55a81318c468,
tupleid=tupleid@entry=0x7ffd54492450, oldtuple=oldtuple@entry=0x0,
slot=<optimized out>, slot@entry=0x55a8131a2f08,
planSlot=planSlot@entry=0x55a81319db60,
epqstate=epqstate@entry=0x55a81318c518, estate=0x55a81318c018, canSetTag=1
'\001') at
/build/postgresql-10-qAeTPy/postgresql-10-10.1/build/../src/backend/executor/nodeModifyTable.c:1113
#6  0x000055a8127480c5 in ExecModifyTable (pstate=0x55a81318c468) at
/build/postgresql-10-qAeTPy/postgresql-10-10.1/build/../src/backend/executor/nodeModifyTable.c:1770
#7  0x000055a81272a28b in ExecProcNode (node=0x55a81318c468) at
/build/postgresql-10-qAeTPy/postgresql-10-10.1/build/../src/include/executor/executor.h:250
#8  ExecutePlan (execute_once=<optimized out>, dest=0x7f4e4e18d7a8,
direction=<optimized out>, numberTuples=0, sendTuples=<optimized out>,
operation=CMD_UPDATE, use_parallel_mode=<optimized out>,
planstate=0x55a81318c468, estate=0x55a81318c018) at
/build/postgresql-10-qAeTPy/postgresql-10-10.1/build/../src/backend/executor/execMain.c:1722
#9  standard_ExecutorRun (queryDesc=0x55a813156838, direction=<optimized
out>, count=0, execute_once=<optimized out>) at
/build/postgresql-10-qAeTPy/postgresql-10-10.1/build/../src/backend/executor/execMain.c:363
#10 0x000055a812864f05 in ProcessQuery (plan=<optimized out>,
sourceText=0x55a8130f3b08 "UPDATE posts\n", ' ' <repeats 24 times>, "SET
avg_time = (x.gmean / 1000)\n", ' ' <repeats 24 times>, "FROM (SELECT
post_timings.topic_id,\n", ' ' <repeats 37 times>,
"post_timings.post_number,\n        "..., params=0x0, queryEnv=0x0,
dest=0x7f4e4e18d7a8, completionTag=0x7ffd544927f0 "") at
/build/postgresql-10-qAeTPy/postgresql-10-10.1/build/../src/backend/tcop/pquery.c:161
#11 0x000055a81286515b in PortalRunMulti
(portal=portal@entry=0x55a81318a008, isTopLevel=isTopLevel@entry=1 '\001',
setHoldSnapshot=setHoldSnapshot@entry=0 '\000',
dest=dest@entry=0x7f4e4e18d7a8, altdest=altdest@entry=0x7f4e4e18d7a8,
completionTag=completionTag@entry=0x7ffd544927f0 "") at
/build/postgresql-10-qAeTPy/postgresql-10-10.1/build/../src/backend/tcop/pquery.c:1286
#12 0x000055a812865e04 in PortalRun (portal=portal@entry=0x55a81318a008,
count=count@entry=9223372036854775807, isTopLevel=isTopLevel@entry=1 '\001',
run_once=run_once@entry=1 '\001', dest=dest@entry=0x7f4e4e18d7a8,
altdest=altdest@entry=0x7f4e4e18d7a8, completionTag=0x7ffd544927f0 "") at
/build/postgresql-10-qAeTPy/postgresql-10-10.1/build/../src/backend/tcop/pquery.c:799
#13 0x000055a8128618dc in exec_simple_query (query_string=0x55a8130f3b08
"UPDATE posts\n", ' ' <repeats 24 times>, "SET avg_time = (x.gmean /
1000)\n", ' ' <repeats 24 times>, "FROM (SELECT post_timings.topic_id,\n", '
' <repeats 37 times>, "post_timings.post_number,\n        "...) at
/build/postgresql-10-qAeTPy/postgresql-10-10.1/build/../src/backend/tcop/postgres.c:1099
#14 0x000055a812862df6 in PostgresMain (argc=<optimized out>,
argv=argv@entry=0x55a81309b738, dbname=0x55a813053b78
"boingboing_discourse", username=<optimized out>) at
/build/postgresql-10-qAeTPy/postgresql-10-10.1/build/../src/backend/tcop/postgres.c:4088
#15 0x000055a81259cfe9 in BackendRun (port=0x55a813092c70) at
/build/postgresql-10-qAeTPy/postgresql-10-10.1/build/../src/backend/postmaster/postmaster.c:4357
#16 BackendStartup (port=0x55a813092c70) at
/build/postgresql-10-qAeTPy/postgresql-10-10.1/build/../src/backend/postmaster/postmaster.c:4029
#17 ServerLoop () at
/build/postgresql-10-qAeTPy/postgresql-10-10.1/build/../src/backend/postmaster/postmaster.c:1753
#18 0x000055a8127f236b in PostmasterMain (argc=3, argv=<optimized out>) at
/build/postgresql-10-qAeTPy/postgresql-10-10.1/build/../src/backend/postmaster/postmaster.c:1361
#19 0x000055a81259e3e5 in main (argc=3, argv=0x55a813051a90) at
/build/postgresql-10-qAeTPy/postgresql-10-10.1/build/../src/backend/main/main.c:228

(gdb) bt full
#0  index_markpos (scan=0x0) at
/build/postgresql-10-qAeTPy/postgresql-10-10.1/build/../src/backend/access/index/indexam.c:373
        __func__ = "index_markpos"
#1  0x000055a812746c68 in ExecMergeJoin (pstate=0x55a8131bc778) at
/build/postgresql-10-qAeTPy/postgresql-10-10.1/build/../src/backend/executor/nodeMergejoin.c:1188
        node = 0x55a8131bc778
        joinqual = 0x55a8131bcac8
        otherqual = 0x0
        qualResult = <optimized out>
        compareResult = <optimized out>
        innerPlan = 0x55a8131c8958
        innerTupleSlot = <optimized out>
        outerPlan = 0x55a8131bdd60
        outerTupleSlot = <optimized out>
        econtext = 0x55a8131bc888
        doFillOuter = 0 '\000'
        doFillInner = 0 '\000'
        __func__ = "ExecMergeJoin"
#2  0x000055a81272cf3f in ExecProcNode (node=0x55a8131bc778) at
/build/postgresql-10-qAeTPy/postgresql-10-10.1/build/../src/include/executor/executor.h:250
No locals.
#3  EvalPlanQualNext (epqstate=epqstate@entry=0x55a81318c518) at
/build/postgresql-10-qAeTPy/postgresql-10-10.1/build/../src/backend/executor/execMain.c:3005
        oldcontext = 0x55a813146198
#4  0x000055a81272d342 in EvalPlanQual (estate=estate@entry=0x55a81318c018,
epqstate=epqstate@entry=0x55a81318c518,
relation=relation@entry=0x7f4e4e25ab68, rti=1, lockmode=<optimized out>,
tid=tid@entry=0x7ffd54492330, priorXmax=8959603) at
/build/postgresql-10-qAeTPy/postgresql-10-10.1/build/../src/backend/executor/execMain.c:2521
        slot = <optimized out>
        copyTuple = 0x7f4e4e167d68
#5  0x000055a812747af7 in ExecUpdate (mtstate=mtstate@entry=0x55a81318c468,
tupleid=tupleid@entry=0x7ffd54492450, oldtuple=oldtuple@entry=0x0,
slot=<optimized out>, slot@entry=0x55a8131a2f08,
planSlot=planSlot@entry=0x55a81319db60,
epqstate=epqstate@entry=0x55a81318c518, estate=0x55a81318c018, canSetTag=1
'\001') at
/build/postgresql-10-qAeTPy/postgresql-10-10.1/build/../src/backend/executor/nodeModifyTable.c:1113
        epqslot = <optimized out>
        lockmode = LockTupleNoKeyExclusive
        tuple = <optimized out>
        resultRelInfo = 0x55a81318c2b8
        resultRelationDesc = 0x7f4e4e25ab68
        result = <optimized out>
        hufd = {ctid = {ip_blkid = {bi_hi = 2, bi_lo = 29090}, ip_posid =
7}, xmax = 8959603, cmax = 4294967295}
        recheckIndexes = 0x0
        __func__ = "ExecUpdate"
#6  0x000055a8127480c5 in ExecModifyTable (pstate=0x55a81318c468) at
/build/postgresql-10-qAeTPy/postgresql-10-10.1/build/../src/backend/executor/nodeModifyTable.c:1770
        node = 0x55a81318c468
        estate = 0x55a81318c018
        operation = CMD_UPDATE
        saved_resultRelInfo = 0x0
        resultRelInfo = 0x55a81318c2b8
        subplanstate = 0x55a81318d238
        junkfilter = 0x55a8131a50a8
        slot = 0x55a8131a2f08
        tupleid = 0x7ffd54492450
        tuple_ctid = {ip_blkid = {bi_hi = 2, bi_lo = 29090}, ip_posid =
11}
        oldtupdata = {t_len = 0, t_self = {ip_blkid = {bi_hi = 0, bi_lo =
0}, ip_posid = 26680}, t_tableOid = 21928, t_data = 0x55a81318c018}
        oldtuple = 0x0
        __func__ = "ExecModifyTable"
#7  0x000055a81272a28b in ExecProcNode (node=0x55a81318c468) at
/build/postgresql-10-qAeTPy/postgresql-10-10.1/build/../src/include/executor/executor.h:250
No locals.
#8  ExecutePlan (execute_once=<optimized out>, dest=0x7f4e4e18d7a8,
direction=<optimized out>, numberTuples=0, sendTuples=<optimized out>,
operation=CMD_UPDATE, use_parallel_mode=<optimized out>,
planstate=0x55a81318c468, estate=0x55a81318c018) at
/build/postgresql-10-qAeTPy/postgresql-10-10.1/build/../src/backend/executor/execMain.c:1722
        slot = <optimized out>
        current_tuple_count = 0
#9  standard_ExecutorRun (queryDesc=0x55a813156838, direction=<optimized
out>, count=0, execute_once=<optimized out>) at
/build/postgresql-10-qAeTPy/postgresql-10-10.1/build/../src/backend/executor/execMain.c:363
        estate = 0x55a81318c018
        operation = CMD_UPDATE
        dest = 0x7f4e4e18d7a8
        sendTuples = <optimized out>
        __func__ = "standard_ExecutorRun"
#10 0x000055a812864f05 in ProcessQuery (plan=<optimized out>,
sourceText=0x55a8130f3b08 "UPDATE posts\n", ' ' <repeats 24 times>, "SET
avg_time = (x.gmean / 1000)\n", ' ' <repeats 24 times>, "FROM (SELECT
post_timings.topic_id,\n", ' ' <repeats 37 times>,
"post_timings.post_number,\n        "..., params=0x0, queryEnv=0x0,
dest=0x7f4e4e18d7a8, completionTag=0x7ffd544927f0 "") at
/build/postgresql-10-qAeTPy/postgresql-10-10.1/build/../src/backend/tcop/pquery.c:161
        queryDesc = 0x55a813156838
#11 0x000055a81286515b in PortalRunMulti
(portal=portal@entry=0x55a81318a008, isTopLevel=isTopLevel@entry=1 '\001',
setHoldSnapshot=setHoldSnapshot@entry=0 '\000',
dest=dest@entry=0x7f4e4e18d7a8, altdest=altdest@entry=0x7f4e4e18d7a8,
completionTag=completionTag@entry=0x7ffd544927f0 "") at
/build/postgresql-10-qAeTPy/postgresql-10-10.1/build/../src/backend/tcop/pquery.c:1286
        pstmt = 0x7f4e4e18d6c8
        active_snapshot_set = 1 '\001'
        stmtlist_item = 0x7f4e4e18d758
#12 0x000055a812865e04 in PortalRun (portal=portal@entry=0x55a81318a008,
count=count@entry=9223372036854775807, isTopLevel=isTopLevel@entry=1 '\001',
run_once=run_once@entry=1 '\001', dest=dest@entry=0x7f4e4e18d7a8,
altdest=altdest@entry=0x7f4e4e18d7a8, completionTag=0x7ffd544927f0 "") at
/build/postgresql-10-qAeTPy/postgresql-10-10.1/build/../src/backend/tcop/pquery.c:799
        save_exception_stack = 0x7ffd54492a20
        save_context_stack = 0x0
        local_sigjmp_buf = {{__jmpbuf = {1, -2410647209184299319,
94180362802640, 140726017533936, 94180363247624, 94180362802688,
-2410647209169619255, -8493467030827796791}, __mask_was_saved = 0,
__saved_mask = {__val = {8192, 0, 0, 12099560782854432544, 0,
94180356443286, 64, 140726017533776, 94180363247624, 88, 94180362802640, 2,
139974294427512, 140726017533808, 94180354932849, 2}}}}
        result = <optimized out>
        nprocessed = <optimized out>
        saveTopTransactionResourceOwner = 0x55a8130a1938
        saveTopTransactionContext = 0x55a8130a1bd8
        saveActivePortal = 0x0
        saveResourceOwner = 0x55a8130a1938
        savePortalContext = 0x0
        saveMemoryContext = 0x55a8130a1bd8
        __func__ = "PortalRun"
#13 0x000055a8128618dc in exec_simple_query (query_string=0x55a8130f3b08
"UPDATE posts\n", ' ' <repeats 24 times>, "SET avg_time = (x.gmean /
1000)\n", ' ' <repeats 24 times>, "FROM (SELECT post_timings.topic_id,\n", '
' <repeats 37 times>, "post_timings.post_number,\n        "...) at
/build/postgresql-10-qAeTPy/postgresql-10-10.1/build/../src/backend/tcop/postgres.c:1099
        parsetree = 0x55a81311d5d0
        portal = 0x55a81318a008
        snapshot_set = <optimized out>
        commandTag = <optimized out>
        completionTag =

"\000(IT\375\177\000\000\001u\230\022\250U\000\000\060(IT\375\177\000\000(\022\346\022\001\000\000\000\234$\336\022\250U\000\000\234$\336\022\250U\000\000\b;\017\023\250U\000\000\001\000\000\000\000\000\000"
        querytree_list = <optimized out>
        plantree_list = <optimized out>
        receiver = 0x7f4e4e18d7a8
        format = 0
        dest = DestRemote
        parsetree_list = 0x55a81311d620
        parsetree_item = 0x55a81311d600
        save_log_statement_stats = 0 '\000'
        was_logged = 0 '\000'
        isTopLevel = 1 '\001'
        msec_str =
"\000(IT\375\177\000\000\001u\230\022\250U\000\000\060(IT\375\177\000\000(\022\346\022\001\000\000"
        __func__ = "exec_simple_query"
#14 0x000055a812862df6 in PostgresMain (argc=<optimized out>,
argv=argv@entry=0x55a81309b738, dbname=0x55a813053b78
"boingboing_discourse", username=<optimized out>) at
/build/postgresql-10-qAeTPy/postgresql-10-10.1/build/../src/backend/tcop/postgres.c:4088
        query_string = 0x55a8130f3b08 "UPDATE posts\n", ' ' <repeats 24
times>, "SET avg_time = (x.gmean / 1000)\n", ' ' <repeats 24 times>, "FROM
(SELECT post_timings.topic_id,\n", ' ' <repeats 37 times>,
"post_timings.post_number,\n        "...
        firstchar = 81
        input_message = {data = 0x55a8130f3b08 "UPDATE posts\n", ' '
<repeats 24 times>, "SET avg_time = (x.gmean / 1000)\n", ' ' <repeats 24
times>, "FROM (SELECT post_timings.topic_id,\n", ' ' <repeats 37 times>,
"post_timings.post_number,\n        "..., len = 942, maxlen = 1024, cursor =
942}
        local_sigjmp_buf = {{__jmpbuf = {140726017534400,
-2410647209056373047, 94180362270520, 1, 94180362270200, 94180362234992,
-2410647208949418295, -8493467028741261623}, __mask_was_saved = 1,
__saved_mask = {__val = {0, 140726017534704, 94180352583639, 94180359415580,
140726017534620, 94180359731456, 94180356159802, 140726017534928, 686, 0,
94180359927584, 94180362234992, 139974255290296, 94180359464352,
206158430256, 140726017534912}}}}
        send_ready_for_query = 0 '\000'
        disable_idle_in_transaction_timeout = <optimized out>
        __func__ = "PostgresMain"
#15 0x000055a81259cfe9 in BackendRun (port=0x55a813092c70) at
/build/postgresql-10-qAeTPy/postgresql-10-10.1/build/../src/backend/postmaster/postmaster.c:4357
        ac = 1
        secs = 570264982
        usecs = 174473
        i = 1
        av = 0x55a81309b738
        maxac = <optimized out>
#16 BackendStartup (port=0x55a813092c70) at
/build/postgresql-10-qAeTPy/postgresql-10-10.1/build/../src/backend/postmaster/postmaster.c:4029
        bn = <optimized out>
        pid = <optimized out>
#17 ServerLoop () at
/build/postgresql-10-qAeTPy/postgresql-10-10.1/build/../src/backend/postmaster/postmaster.c:1753
        rmask = {fds_bits = {64, 0 <repeats 15 times>}}
        selres = <optimized out>
        now = <optimized out>
        readmask = {fds_bits = {112, 0 <repeats 15 times>}}
        last_lockfile_recheck_time = 1516949750
        last_touch_time = 1516949630
        __func__ = "ServerLoop"
#18 0x000055a8127f236b in PostmasterMain (argc=3, argv=<optimized out>) at
/build/postgresql-10-qAeTPy/postgresql-10-10.1/build/../src/backend/postmaster/postmaster.c:1361
        opt = <optimized out>
        status = <optimized out>
        userDoption = <optimized out>
        listen_addr_saved = 1 '\001'
        i = <optimized out>
        output_config_variable = <optimized out>
        __func__ = "PostmasterMain"
#19 0x000055a81259e3e5 in main (argc=3, argv=0x55a813051a90) at
/build/postgresql-10-qAeTPy/postgresql-10-10.1/build/../src/backend/main/main.c:228
No locals.
```

## Configuration changes

```
shared_buffers = "1GB"
work_mem = "16MB"
maintenance_work_mem = "16MB"
``` 


Do feel free to let me know if I'm required to provide more information and
I'll be happy to do so.