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  1. Fix contrib/auto_explain to not cause problems in parallel workers.

  1. BUG #15821: Parallel Workers with functions and auto_explain: ERROR: could not find key 3 in shm TOC

    The Post Office <noreply@postgresql.org> — 2019-05-27T16:11:37Z

    The following bug has been logged on the website:

Bug reference:      15821
Logged by:          Christian Hofstaedtler
Email address:      ch+pg@zeha.at
PostgreSQL version: 11.3
Operating system:   Debian stretch amd64
Description:        

We have enabled auto_explain and see errors on PostgreSQL 11.3 when
SELECTing from a user defined function. No such crashes have been
observed on 10.7.

Maybe relevant config settings:
  log_min_duration_statement = 0
  auto_explain.log_min_duration = 150ms
  auto_explain.log_analyze = on
  shared_preload_libraries = 'pg_stat_statements'
  pg_stat_statements.track = all
  pg_stat_statements.max = 10000

I can trigger the error on our database using:

  CREATE TABLE reprotable(id serial, val int);
  INSERT INTO reprotable(val) SELECT * FROM generate_series(1, 10000000);
  CREATE OR REPLACE FUNCTION public.crashrepro4() RETURNS TABLE(foo integer,
foo2 integer) LANGUAGE sql AS $function$
  SELECT c.id, v.val FROM reprotable c JOIN reprotable v ON v.id = c.id
  $function$;
  
  LOAD 'auto_explain';
  SET max_parallel_workers_per_gather TO 8;
  SELECT * FROM crashrepro4();

Sometimes this works on the first few tries; running
  EXPLAIN ANALYZE SELECT * FROM crashrepro4();
appears to help with the reproduction.

Output:
ERROR:  could not find key 3 in shm TOC at 0x7f45a0334000
CONTEXT:  parallel worker
SQL function "crashrepro4" statement 1

Backtrace:

Program received signal SIGUSR1, User defined signal 1.
0x00007f459dc94bc6 in posix_fallocate64 () from
/lib/x86_64-linux-gnu/libc.so.6
(gdb) bt full
#0  0x00007f459dc94bc6 in posix_fallocate64 () from
/lib/x86_64-linux-gnu/libc.so.6
No symbol table info available.
#1  0x00005636d9cc675b in dsm_impl_posix_resize (size=134483968, fd=7) at
./build/../src/backend/storage/ipc/dsm_impl.c:441
        rc = <optimized out>
#2  dsm_impl_posix (impl_private=0x8041000, elevel=20,
mapped_size=0x5636dc084348, mapped_address=0x5636dc084340,
request_size=134483968, handle=<optimized out>, op=DSM_OP_CREATE) at
./build/../src/backend/storage/ipc/dsm_impl.c:326
        flags = <optimized out>
        fd = 7
        name =
"/PostgreSQL.1895625775\000\000X\274ʝE\177\000\000@\354\322\357\377\177\000\000\020\337\003\334\066V\000\000@\000\000\000\000\000\000\000\000\200\000\000\000\000\000"
        address = <optimized out>
#3  dsm_impl_op (op=op@entry=DSM_OP_CREATE, handle=<optimized out>,
request_size=request_size@entry=134483968,
impl_private=impl_private@entry=0x5636dc084338,
mapped_address=mapped_address@entry=0x5636dc084340, 
    mapped_size=mapped_size@entry=0x5636dc084348, elevel=20) at
./build/../src/backend/storage/ipc/dsm_impl.c:177
        __func__ = "dsm_impl_op"
#4  0x00005636d9cc7877 in dsm_create (size=size@entry=134483968,
flags=flags@entry=0) at ./build/../src/backend/storage/ipc/dsm.c:474
        seg = 0x5636dc084318
        i = <optimized out>
        nitems = <optimized out>
        __func__ = "dsm_create"
#5  0x00005636d9e2b317 in make_new_segment (area=area@entry=0x5636dc17fd08,
requested_pages=requested_pages@entry=32768) at
./build/../src/backend/utils/mmgr/dsa.c:2155
        new_index = 2
        metadata_bytes = 266240
        total_size = 134483968
        total_pages = <optimized out>
        usable_pages = 32768
        segment_map = <optimized out>
        segment = <optimized out>
#6  0x00005636d9e2cbce in dsa_allocate_extended (area=0x5636dc17fd08,
size=size@entry=134217728, flags=flags@entry=0) at
./build/../src/backend/utils/mmgr/dsa.c:712
        npages = 32768
        first_page = 139937017269144
        pool = 0x7f45a02a25f0
        size_class = <optimized out>
        start_pointer = <optimized out>
        segment_map = <optimized out>
        __func__ = "dsa_allocate_extended"
#7  0x00005636d9bb3e95 in ExecParallelHashTableAlloc
(hashtable=hashtable@entry=0x5636dc173a70, batchno=batchno@entry=0) at
./build/../src/backend/executor/nodeHash.c:3047
        batch = 0x7f417ccc0000
        buckets = <optimized out>
        nbuckets = 16777216
        i = <optimized out>
#8  0x00005636d9bb42e3 in ExecHashTableCreate
(state=state@entry=0x5636dc146280, hashOperators=<optimized out>,
keepNulls=<optimized out>) at
./build/../src/backend/executor/nodeHash.c:615
        pstate = <optimized out>
        build_barrier = 0x7f45a02a1450
        node = 0x5636dc140568
        hashtable = 0x5636dc173a70
        outerNode = <optimized out>
        space_allowed = 64424509440
        nbuckets = 16777216
        nbatch = 1
        rows = <optimized out>
        num_skew_mcvs = 10391049
        log2_nbuckets = <optimized out>
        i = 0
        ho = <optimized out>
        __func__ = "ExecHashTableCreate"
#9  0x00005636d9bb7085 in ExecHashJoinImpl (parallel=true, pstate=<optimized
out>) at ./build/../src/backend/executor/nodeHashjoin.c:279
        outerNode = <optimized out>
        hashNode = <optimized out>
        econtext = <optimized out>
        node = <optimized out>
        joinqual = <optimized out>
        otherqual = <optimized out>
        hashtable = 0x0
        hashvalue = 32767
        batchno = -641936805
        parallel_state = <optimized out>
#10 ExecParallelHashJoin (pstate=<optimized out>) at
./build/../src/backend/executor/nodeHashjoin.c:581
No locals.
#11 0x00005636d9bb2194 in ExecProcNode (node=0x5636dc145b60) at
./build/../src/include/executor/executor.h:247
No locals.
#12 gather_getnext (gatherstate=0x5636dc145970) at
./build/../src/backend/executor/nodeGather.c:276
        estate = 0x5636dc145730
        outerPlan = 0x5636dc145b60
        fslot = 0x5636dc0743a8
#13 ExecGather (pstate=0x5636dc145970) at
./build/../src/backend/executor/nodeGather.c:207
        node = 0x5636dc145970
        econtext = 0x5636dc145a80
#14 0x00005636d9b9b4c3 in ExecProcNode (node=0x5636dc145970) at
./build/../src/include/executor/executor.h:247
No locals.
#15 ExecutePlan (execute_once=<optimized out>, dest=0x5636dc141648,
direction=<optimized out>, numberTuples=0, sendTuples=<optimized out>,
operation=CMD_SELECT, use_parallel_mode=<optimized out>,
planstate=0x5636dc145970, 
    estate=0x5636dc145730) at
./build/../src/backend/executor/execMain.c:1723
        slot = <optimized out>
        current_tuple_count = 0
#16 standard_ExecutorRun (queryDesc=0x5636dc141698, direction=<optimized
out>, count=0, execute_once=<optimized out>) at
./build/../src/backend/executor/execMain.c:364
        estate = 0x5636dc145730
        operation = CMD_SELECT
        dest = 0x5636dc141648
        sendTuples = <optimized out>
        __func__ = "standard_ExecutorRun"
#17 0x00007f4597d08e15 in pgss_ExecutorRun (queryDesc=0x5636dc141698,
direction=ForwardScanDirection, count=0, execute_once=<optimized out>) at
./build/../contrib/pg_stat_statements/pg_stat_statements.c:892
        save_exception_stack = 0x7fffefd2f0a0
        save_context_stack = 0x7fffefd2f1d0
        local_sigjmp_buf = {{__jmpbuf = {94793620402592, -81779228135919821,
1, 0, 94793620523992, 94793620526040, -81779228572127437,
-24340724960596173}, __mask_was_saved = 0, __saved_mask = {__val =
{94793619686312, 
                140737216966656, 94793581381244, 94793619686312,
94793620543280, 140737216966688, 94793581092645, 94793620543856,
94793620543280, 140737216966736, 94793581143132, 94793620522984, 16, 0,
94793620543280, 
                140737216966784}}}}
#18 0x00007f417cdbf4dd in explain_ExecutorRun (queryDesc=0x5636dc141698,
direction=ForwardScanDirection, count=0, execute_once=<optimized out>) at
./build/../contrib/auto_explain/auto_explain.c:268
        save_exception_stack = 0x7fffefd2f8a0
        save_context_stack = 0x7fffefd2f1d0
        local_sigjmp_buf = {{__jmpbuf = {94793620402592, -81779228104462541,
1, 0, 94793620523992, 94793620526040, -81779228138016973,
-26933449381777613}, __mask_was_saved = 0, __saved_mask = {__val =
{94793581107904, 94793620526744, 
                94793588700640, 140737216966960, 139936876507429,
140737216967008, 94793620526744, 1, 0, 140737216967024, 139919244391853, 1,
0, 94793620523992, 94793620402592, 1}}}}
#19 0x00005636d9ba8f6a in postquel_getnext (es=0x5636dc140bd8,
es=0x5636dc140bd8, fcache=0x5636dc1231a0, fcache=0x5636dc1231a0) at
./build/../src/backend/executor/functions.c:867
        count = 0
#20 fmgr_sql (fcinfo=0x7fffefd2f340) at
./build/../src/backend/executor/functions.c:1164
        fcache = 0x5636dc1231a0
        sqlerrcontext = {previous = 0x0, callback = 0x5636d9ba77a0
<sql_exec_error_callback>, arg = 0x5636dc06a230}
        randomAccess = false
        lazyEvalOK = <optimized out>
        is_first = <optimized out>
        pushed_snapshot = true
        es = 0x5636dc140bd8
        slot = <optimized out>
        result = <optimized out>
        eslist = <optimized out>
        eslc = 0x5636dc1413d8
        __func__ = "fmgr_sql"
#21 0x00005636d9ba4bac in ExecMakeTableFunctionResult
(setexpr=0x5636dc06a210, econtext=0x5636dc06a0e0, argContext=<optimized
out>, expectedDesc=0x5636dc06ac68, randomAccess=false) at
./build/../src/backend/executor/execSRF.c:231
        result = 94793580413993
        tupstore = 0x0
        tupdesc = 0x0
        funcrettype = 2249
        returnsTuple = <optimized out>
        returnsSet = true
        fcinfo = {flinfo = 0x5636dc06a230, context = 0x0, resultinfo =
0x7fffefd2f300, fncollation = 0, isnull = false, nargs = 0, arg =
{94793619488832, 140733193388046, 0, 94793580418128, 140737216967520, 6,
3432, 140733193388035, 
            94793585267779, 1, 59588104878912, 94793585267784, 32,
139936976061625, 94793584837154, 1, 140737216968608, 3, 140737216967832,
206158430224, 140737216969520, 94793620609964, 94793620609964,
139936977225618, 
            139936979540000, 8, 0, 139936976486110, 3688503315210960912,
8187, 0, 94793620609936, 94793620618144, 139936979544832, 0, 160, 142,
139936976269588, 140737216967904, 94793583475689, 140737216968048,
94793620609952, 
            94793620609936, 252098579, 140737216968464, 139937017242608,
139937017242584, 139937017242608, 139937017242608, 94793619743656,
140737216967952, 94793583769478, 139937017242608, 7074228167464498944,
94793620438296, 
            94793581565312, 140737216968032, 94793581372751,
140737216968032, 94793620438296, 140737216968464, 140737216968464,
94793584972684, 0, 140737216968144, 94793581565681, 140737216968080,
7074228167464498944, 94793620438216, 
            94793581565312, 140737216968144, 94793581372751, 1,
7074228167464498944, 140737216968464, 94793620438216, 140737216968464,
140737216968464, 140737216968256, 94793581565681, 4294967295,
139937017247848, 139937017247824, 
            139937017247848, 139937017247848, 94793619743656,
140737216968240, 7074228167464498944, 139937017247848, 94793620438376,
94793581565312, 140737216968464, 140737216968320, 94793581372827,
140737216968304, 94793620438408, 
            140737216968464, 94793620436824, 0, 0}, argnull = {240, 246,
210, 239, 255, 127, false, false, 241, 150, 193, 217, 54, 86, false, false,
16, 247, 210, 239, 255, 127, false, false, 169, 156, 193, 217, 54, 86,
false, false, 
            240, 246, 210, 239, 255, 127, false, false, 233, 187, 222, 217,
54, 86, false, false, 24, false, false, false, false, false, false, false,
175, 189, 160, 217, 54, 86, false, false, 16, 247, 210, 239, 255, 127,
false, 
            false, 233, 187, 222, 217, 54, 86, false, false, 23, false,
false, false, false, false, false, false, 88, 171, 6, 220, 54, 86, false,
false, 80, 56, 52, 160}}
        fcusage = {fs = 0x5636dc1278c0, save_f_total_time = {tv_sec = 0,
tv_nsec = 0}, save_total = {tv_sec = 0, tv_nsec = 0}, f_start = {tv_sec =
469579, tv_nsec = 359852825}}
        rsinfo = {type = T_ReturnSetInfo, econtext = 0x5636dc06a0e0,
expectedDesc = 0x5636dc06ac68, allowedModes = 11, returnMode =
SFRM_ValuePerCall, isDone = ExprSingleResult, setResult = 0x0, setDesc =
0x0}
        tmptup = {t_len = 0, t_self = {ip_blkid = {bi_hi = 0, bi_lo = 0},
ip_posid = 2}, t_tableOid = 0, t_data = 0x2}
        callerContext = 0x5636dc069cb0
        first_time = true
        __func__ = "ExecMakeTableFunctionResult"
#22 0x00005636d9bb1760 in FunctionNext (node=node@entry=0x5636dc069fd0) at
./build/../src/backend/executor/nodeFunctionscan.c:94
        tstore = 0x0
        estate = <optimized out>
        direction = ForwardScanDirection
        scanslot = 0x5636dc06ae88
        alldone = <optimized out>
        oldpos = <optimized out>
        funcno = <optimized out>
        att = <optimized out>
#23 0x00005636d9ba40f9 in ExecScanFetch (recheckMtd=0x5636d9bb14a0
<FunctionRecheck>, accessMtd=0x5636d9bb14d0 <FunctionNext>,
node=0x5636dc069fd0) at ./build/../src/backend/executor/execScan.c:95
        estate = 0x5636dc069dc0
#24 ExecScan (node=0x5636dc069fd0, accessMtd=0x5636d9bb14d0 <FunctionNext>,
recheckMtd=0x5636d9bb14a0 <FunctionRecheck>) at
./build/../src/backend/executor/execScan.c:145
        econtext = <optimized out>
        qual = 0x0
        projInfo = 0x0
#25 0x00005636d9ba25c9 in ExecProcNodeInstr (node=0x5636dc069fd0) at
./build/../src/backend/executor/execProcnode.c:461
        result = <optimized out>
#26 0x00005636d9b9b4c3 in ExecProcNode (node=0x5636dc069fd0) at
./build/../src/include/executor/executor.h:247
No locals.
#27 ExecutePlan (execute_once=<optimized out>, dest=0x5636dc12ccb8,
direction=<optimized out>, numberTuples=0, sendTuples=<optimized out>,
operation=CMD_SELECT, use_parallel_mode=<optimized out>,
planstate=0x5636dc069fd0, 
    estate=0x5636dc069dc0) at
./build/../src/backend/executor/execMain.c:1723
        slot = <optimized out>
        current_tuple_count = 0
#28 standard_ExecutorRun (queryDesc=0x5636dc068590, direction=<optimized
out>, count=0, execute_once=<optimized out>) at
./build/../src/backend/executor/execMain.c:364
        estate = 0x5636dc069dc0
        operation = CMD_SELECT
        dest = 0x5636dc12ccb8
        sendTuples = <optimized out>
        __func__ = "standard_ExecutorRun"
#29 0x00007f4597d08e15 in pgss_ExecutorRun (queryDesc=0x5636dc068590,
direction=ForwardScanDirection, count=0, execute_once=<optimized out>) at
./build/../contrib/pg_stat_statements/pg_stat_statements.c:892
        save_exception_stack = 0x7fffefd2f9b0
        save_context_stack = 0x0
        local_sigjmp_buf = {{__jmpbuf = {94793620008672, -81779227836027085,
0, 94793619637648, 0, 1, -81779227869581517, -24340724960596173},
__mask_was_saved = 0, __saved_mask = {__val = {94793583733915,
140737216969008, 
                7074228167464498944, 140737216969008, 94793619643840,
94793620438296, 94793619644864, 2, 94793619644368, 140737216969168,
94793581140788, 7987184768, 94793620441608, 94793619643840, 336, 1}}}}
#30 0x00007f417cdbf4dd in explain_ExecutorRun (queryDesc=0x5636dc068590,
direction=ForwardScanDirection, count=0, execute_once=<optimized out>) at
./build/../contrib/auto_explain/auto_explain.c:268
        save_exception_stack = 0x7fffefd2fb50
        save_context_stack = 0x0
        local_sigjmp_buf = {{__jmpbuf = {94793620008672, -81779227930398925,
0, 94793619637648, 0, 1, -81779227829735629, -26933449381777613},
__mask_was_saved = 0, __saved_mask = {__val = {94793620441784,
140737216969344, 336, 1, 
                94793620155408, 24, 94793619637648, 0, 1, 140737216969312,
94793583755684, 94793620008672, 94793620155728, 140737216969344,
94793583827107, 94793620008672}}}}
#31 0x00005636d9cee59b in PortalRunSelect
(portal=portal@entry=0x5636dc0c2ee0, forward=forward@entry=true, count=0,
count@entry=9223372036854775807, dest=dest@entry=0x5636dc12ccb8) at
./build/../src/backend/tcop/pquery.c:932
        queryDesc = 0x5636dc068590
        direction = <optimized out>
        nprocessed = <optimized out>
        __func__ = "PortalRunSelect"
#32 0x00005636d9cefb20 in PortalRun (portal=portal@entry=0x5636dc0c2ee0,
count=count@entry=9223372036854775807, isTopLevel=isTopLevel@entry=true,
run_once=run_once@entry=true, dest=dest@entry=0x5636dc12ccb8, 
    altdest=altdest@entry=0x5636dc12ccb8, completionTag=0x7fffefd2fca0 "")
at ./build/../src/backend/tcop/pquery.c:773
        save_exception_stack = 0x7fffefd2fec0
        save_context_stack = 0x0
        local_sigjmp_buf = {{__jmpbuf = {1, -81779228012187853,
94793619488704, 94793619488752, 94793620008672, 2, -81779227917816013,
-5958515187191648461}, __mask_was_saved = 0, __saved_mask = {__val =
{94793619463952, 0, 
                2817148525, 94793620020232, 94793619637376, 4, 112,
94793619488704, 94793619484688, 94793584837140, 2, 140737216969760,
94793583757029, 140737216969792, 2, 94793619488704}}}}
        result = <optimized out>
        nprocessed = <optimized out>
        saveTopTransactionResourceOwner = 0x5636dc0823a8
        saveTopTransactionContext = 0x5636dc0e6c10
        saveActivePortal = 0x0
        saveResourceOwner = 0x5636dc0823a8
        savePortalContext = 0x0
        saveMemoryContext = 0x5636dc0e6c10
        __func__ = "PortalRun"
#33 0x00005636d9ceb7b9 in exec_simple_query (query_string=0x5636dc043120
"SELECT * FROM crashrepro4();") at
./build/../src/backend/tcop/postgres.c:1145
        parsetree = 0x5636dc043fc0
        portal = 0x5636dc0c2ee0
        snapshot_set = <optimized out>
        commandTag = <optimized out>
        completionTag =
"\000\000\000\000\002\000\000\000\340\374\322\357\377\177\000\000-\r\324\357\377\177\000\000\000\000\000\000\000\000\000\000\360\374\322\357\002\000\000\000Q\000\000\000\000\000\000\000
1\004\334\066V\000\000`\376\322\357\377\177\000"
        querytree_list = <optimized out>
        plantree_list = <optimized out>
        receiver = 0x5636dc12ccb8
        format = 0
        dest = DestRemote
        parsetree_list = 0x5636dc044010
        parsetree_item = 0x5636dc043ff0
        save_log_statement_stats = false
        was_logged = false
        use_implicit_block = false
        msec_str =
"\000\000\000\000\002\000\000\000\340\374\322\357\377\177\000\000-\r\324\357\377\177\000\000\000\000\000\000\000\000\000"
        __func__ = "exec_simple_query"
#34 0x00005636d9ced623 in PostgresMain (argc=<optimized out>,
argv=argv@entry=0x5636dc070128, dbname=<optimized out>, username=<optimized
out>) at ./build/../src/backend/tcop/postgres.c:4182
        query_string = 0x5636dc043120 "SELECT * FROM crashrepro4();"
        input_message = {data = 0x5636dc043120 "SELECT * FROM
crashrepro4();", len = 29, maxlen = 1024, cursor = 29}
        local_sigjmp_buf = {{__jmpbuf = {140737216970368,
-81779225453662413, 1, 94793619668968, 94793619668968, 94793619632848,
-81779227978633421, -5958515189572833485}, __mask_was_saved = 1,
__saved_mask = {__val = {0, 
                140737216970544, 140737216970540, 140737216970640,
8589934592, 94793588696280, 94793585032905, 140737216970864,
140737216972320, 140737216971248, 94793619668968, 94793619632848,
139936976780376, 5, 206158430256, 
                140737216970848}}}}
        send_ready_for_query = false
        disable_idle_in_transaction_timeout = false
        __func__ = "PostgresMain"
#35 0x00005636d99fb097 in BackendRun (port=0x5636dc0672d0) at
./build/../src/backend/postmaster/postmaster.c:4358
        ac = 1
        secs = 612288553
        usecs = 224860
        i = 1
        av = 0x5636dc070128
        maxac = <optimized out>
#36 BackendStartup (port=0x5636dc0672d0) at
./build/../src/backend/postmaster/postmaster.c:4030
        bn = <optimized out>
        pid = <optimized out>
#37 ServerLoop () at ./build/../src/backend/postmaster/postmaster.c:1707
        rmask = {fds_bits = {128, 0 <repeats 15 times>}}
        selres = <optimized out>
        now = <optimized out>
        readmask = {fds_bits = {200, 0 <repeats 15 times>}}
        last_lockfile_recheck_time = 1558973336
        last_touch_time = 1558973276
        __func__ = "ServerLoop"
#38 0x00005636d9c78221 in PostmasterMain (argc=7, argv=0x5636dc03de10) at
./build/../src/backend/postmaster/postmaster.c:1380
        opt = <optimized out>
        status = <optimized out>
        userDoption = <optimized out>
        listen_addr_saved = true
        i = <optimized out>
        output_config_variable = <optimized out>
        __func__ = "PostmasterMain"
#39 0x00005636d99fc594 in main (argc=7, argv=0x5636dc03de10) at
./build/../src/backend/main/main.c:228
No locals.
(gdb)

  2. Re: BUG #15821: Parallel Workers with functions and auto_explain: ERROR: could not find key 3 in shm TOC

    Tom Lane <tgl@sss.pgh.pa.us> — 2019-06-03T19:21:01Z

    PG Bug reporting form <noreply@postgresql.org> writes:
> We have enabled auto_explain and see errors on PostgreSQL 11.3 when
> SELECTing from a user defined function. No such crashes have been
> observed on 10.7.

I think that you didn't give a complete dump of relevant settings,
but after some fooling around I was able to reproduce this error,
and the cause is this: auto_explain hasn't a single clue about
parallel query.

1. In the parent process, we have a parallelizable hash join being
executed in a statement inside a function.  Since
auto_explain.log_nested_statements is not enabled, auto_explain
does not deem that it should trace the statement, so the query
starts up with estate->es_instrument = 0, and therefore
ExecHashInitializeDSM chooses not to create any shared
SharedHashInfo area.

2. In the worker processes, auto_explain manages to grab execution
control when ParallelQueryMain calls ExecutorStart, thanks to being
in ExecutorStart_hook.  Having no clue what's going on, it decides
that this is a new top-level query that it should trace, and it
sets some bits in queryDesc->instrument_options.

3. When the workers get to ExecHashInitializeWorker, they see that
instrumentation is active so they try to look up the SharedHashInfo.
Kaboom.

I'm inclined to think that explain_ExecutorStart should simply
keep its hands off of everything when in a parallel worker;
if instrumentation is required, that'll be indicated by options
passed down from the parent process.  It looks like this could
conveniently be merged with the rate-sampling logic by forcing
current_query_sampled to false when IsParallelWorker().

Likely this should be back-patched all the way to 9.6.  I'm
not sure how we managed to avoid noticing it before now,
but there are probably ways to cause visible trouble in
any release that has any parallel query support.

			regards, tom lane