Re: Parallel stats in execution plans

От: David Conlin
Тема: Re: Parallel stats in execution plans
Дата: ,
Msg-id: 62ed1161-859a-6bc6-c263-a6e2b74f670e@cantab.net
(см: обсуждение, исходный текст)
Ответ на: Parallel stats in execution plans  (David Conlin)
Список: pgsql-performance

It seems like no-one has any ideas on this - does anyone know anywhere else I can try to look/ask to find out more?

Is it possible that this is a bug?

Thanks

Dave

On 16/01/2019 11:31, David Conlin wrote:

Hi folks -

I'm having trouble understanding what some of the stats mean in the execution plan output when parallel workers are used. I've tried to read up about it, but I haven't been able to find anything that explains what I'm seeing. Apologies in advance if there's documentation I've been too stupid to find.

I've run the following query. The "towns" table is a massive table that I created in order to get some big numbers on a parallel query - don't worry, this isn't a real query I want to make faster, just a silly example I'd like to understand.

EXPLAIN (ANALYZE, FORMAT JSON, BUFFERS, VERBOSE)
SELECT
 name, code, article
FROM
 towns
ORDER BY
 name ASC,code DESC;

The output looks like this:

[ {   "Plan": {     "Node Type": "Gather Merge",
      "Parallel Aware": false,
      "Startup Cost": 1013948.54,
      "Total Cost": 1986244.55,
      "Plan Rows": 8333384,
      "Plan Width": 77,
      "Actual Startup Time": 42978.838,
      "Actual Total Time": 60628.982,
      "Actual Rows": 10000010,
      "Actual Loops": 1,
      "Output": ["name", "code", "article"],
      "Workers Planned": 2,
      "Workers Launched": 2,
      "Shared Hit Blocks": 29,
      "Shared Read Blocks": 47641,
      "Shared Dirtied Blocks": 0,
      "Shared Written Blocks": 0,
      "Local Hit Blocks": 0,
      "Local Read Blocks": 0,
      "Local Dirtied Blocks": 0,
      "Local Written Blocks": 0,
      "Temp Read Blocks": 91342,
      "Temp Written Blocks": 91479,
      "Plans": [       {         "Node Type": "Sort",
          "Parent Relationship": "Outer",
          "Parallel Aware": false,
          "Startup Cost": 1012948.52,
          "Total Cost": 1023365.25,
          "Plan Rows": 4166692,
          "Plan Width": 77,
          "Actual Startup Time": 42765.496,
          "Actual Total Time": 48526.168,
          "Actual Rows": 3333337,
          "Actual Loops": 3,
          "Output": ["name", "code", "article"],
          "Sort Key": ["towns.name", "towns.code DESC"],
          "Sort Method": "external merge",
          "Sort Space Used": 283856,
          "Sort Space Type": "Disk",
          "Shared Hit Blocks": 170,
          "Shared Read Blocks": 142762,
          "Shared Dirtied Blocks": 0,
          "Shared Written Blocks": 0,
          "Local Hit Blocks": 0,
          "Local Read Blocks": 0,
          "Local Dirtied Blocks": 0,
          "Local Written Blocks": 0,
          "Temp Read Blocks": 273289,
          "Temp Written Blocks": 273700,
          "Workers": [           {             "Worker Number": 0,
              "Actual Startup Time": 42588.662,
              "Actual Total Time": 48456.662,
              "Actual Rows": 3277980,
              "Actual Loops": 1,
              "Shared Hit Blocks": 72,
              "Shared Read Blocks": 46794,
              "Shared Dirtied Blocks": 0,
              "Shared Written Blocks": 0,
              "Local Hit Blocks": 0,
              "Local Read Blocks": 0,
              "Local Dirtied Blocks": 0,
              "Local Written Blocks": 0,
              "Temp Read Blocks": 89067,
              "Temp Written Blocks": 89202
            },
            {             "Worker Number": 1,
              "Actual Startup Time": 42946.705,
              "Actual Total Time": 48799.414,
              "Actual Rows": 3385130,
              "Actual Loops": 1,
              "Shared Hit Blocks": 69,
              "Shared Read Blocks": 48327,
              "Shared Dirtied Blocks": 0,
              "Shared Written Blocks": 0,
              "Local Hit Blocks": 0,
              "Local Read Blocks": 0,
              "Local Dirtied Blocks": 0,
              "Local Written Blocks": 0,
              "Temp Read Blocks": 92880,
              "Temp Written Blocks": 93019
            }         ],
          "Plans": [           {             "Node Type": "Seq Scan",
              "Parent Relationship": "Outer",
              "Parallel Aware": true,
              "Relation Name": "towns",
              "Schema": "public",
              "Alias": "towns",
              "Startup Cost": 0.00,
              "Total Cost": 184524.92,
              "Plan Rows": 4166692,
              "Plan Width": 77,
              "Actual Startup Time": 0.322,
              "Actual Total Time": 8305.886,
              "Actual Rows": 3333337,
              "Actual Loops": 3,
              "Output": ["name", "code", "article"],
              "Shared Hit Blocks": 96,
              "Shared Read Blocks": 142762,
              "Shared Dirtied Blocks": 0,
              "Shared Written Blocks": 0,
              "Local Hit Blocks": 0,
              "Local Read Blocks": 0,
              "Local Dirtied Blocks": 0,
              "Local Written Blocks": 0,
              "Temp Read Blocks": 0,
              "Temp Written Blocks": 0,
              "Workers": [               {                 "Worker Number": 0,
                  "Actual Startup Time": 0.105,
                  "Actual Total Time": 8394.629,
                  "Actual Rows": 3277980,
                  "Actual Loops": 1,
                  "Shared Hit Blocks": 35,
                  "Shared Read Blocks": 46794,
                  "Shared Dirtied Blocks": 0,
                  "Shared Written Blocks": 0,
                  "Local Hit Blocks": 0,
                  "Local Read Blocks": 0,
                  "Local Dirtied Blocks": 0,
                  "Local Written Blocks": 0,
                  "Temp Read Blocks": 0,
                  "Temp Written Blocks": 0
                },
                {                 "Worker Number": 1,
                  "Actual Startup Time": 0.113,
                  "Actual Total Time": 8139.382,
                  "Actual Rows": 3385130,
                  "Actual Loops": 1,
                  "Shared Hit Blocks": 32,
                  "Shared Read Blocks": 48327,
                  "Shared Dirtied Blocks": 0,
                  "Shared Written Blocks": 0,
                  "Local Hit Blocks": 0,
                  "Local Read Blocks": 0,
                  "Local Dirtied Blocks": 0,
                  "Local Written Blocks": 0,
                  "Temp Read Blocks": 0,
                  "Temp Written Blocks": 0
                }             ]           }         ]       }     ]   },
    "Planning Time": 22.898,
    "Triggers": [   ],
    "Execution Time": 61133.161
  }
]

Or a more slimmed-down version, with just the confusing fields:

[ {   "Plan": {     "Node Type": "Gather Merge",
      "Parallel Aware": false,
      "Actual Total Time": 60628.982,
      "Actual Rows": 10000010,
      "Actual Loops": 1,
      "Workers Planned": 2,
      "Workers Launched": 2,
      "Plans": [       {         "Node Type": "Sort",
          "Parent Relationship": "Outer",
          "Parallel Aware": false,
          "Actual Total Time": 48526.168,
          "Actual Rows": 3333337,
          "Actual Loops": 3,
          "Workers": [           {             "Worker Number": 0,
              "Actual Total Time": 48456.662,
              "Actual Rows": 3277980,
              "Actual Loops": 1
            },
            {             "Worker Number": 1,
              "Actual Total Time": 48799.414,
              "Actual Rows": 3385130,
              "Actual Loops": 1
            }         ],
          "Plans": [           {             "Node Type": "Seq Scan",
              "Parallel Aware": true,
              "Actual Total Time": 8305.886,
              "Actual Rows": 3333337,
              "Actual Loops": 3,
              "Workers": [               {                 "Worker Number": 0,
                  "Actual Total Time": 8394.629,
                  "Actual Rows": 3277980,
                  "Actual Loops": 1
                },
                {                 "Worker Number": 1,
                  "Actual Total Time": 8139.382,
                  "Actual Rows": 3385130,
                  "Actual Loops": 1
                }             ]           }         ]       }     ]   },
    "Execution Time": 61133.161
  }
]

The things I'm struggling to understand are:

  • How the time values combine with parallelism. For example, each execution of the sort node takes an average of 48.5s, over three loops. This makes a total running time of 145.5s. Even if this was perfectly distributed between the two workers, I would expect this to take 72.75s, which is more than the total execution time, so it can't take this long.
  •  How the row numbers combine with those in the "Workers" subkey. For example, in the Sort node, worker #0 produces 3,277,980 rows, while worker #1 produces 3,385,130 rows. The Sort node as a whole produces 3,333,337 rows per loop, for a total of 10,000,010 (the value in the gather merge node). I would have expected the number of rows produced by the two workers to sum to the number produced by the Sort node as a whole, either per loop or in total.
  • How the "Actual Loops" values combine with those in the "Workers" subkey. For example, the "Sort" node has 3 loops, but each of the workers inside it have 1 loop. I would have expected either:
    • each of the workers to have done 3 loops (since the sort is executed 3 times), or
    • the number of loops in the two workers to sum to three (if the three executions of the sort are distributed across the two workers)

Other info about my setup:

  • Postgres version: "PostgreSQL 10.4, compiled by Visual C++ build 1800, 64-bit"
  • I installed Postgres using the EnterpriseDB one-click installer
  • OS: Windows 10, v1803
  • I'm using Jetbrains Datagrip to connect to Postgres
  • No errors are logged.
  • Altered config settings:


  • application_namePostgreSQL JDBC Driversession
    client_encodingUTF8client
    DateStyleISO, MDYclient
    default_text_search_configpg_catalog.englishconfiguration file
    dynamic_shared_memory_typewindowsconfiguration file
    extra_float_digits3session
    lc_messagesEnglish_United States.1252configuration file
    lc_monetaryEnglish_United States.1252configuration file
    lc_numericEnglish_United States.1252configuration file
    lc_timeEnglish_United States.1252configuration file
    listen_addresses*configuration file
    log_destinationstderrconfiguration file
    logging_collectoronconfiguration file
    max_connections100configuration file
    max_stack_depth2MBenvironment variable
    port5432configuration file
    shared_buffers128MBconfiguration file
    TimeZoneUTCclient

Thanks in advance, I'm sure I've done something silly or misunderstood something obvious but I can't work out what it is for the life of me.

Dave


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