Parallel stats in execution plans

[David Conlin <dc345@xxxxxxxxxx>]

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_name	PostgreSQL JDBC Driver	session
  client_encoding	UTF8	client
  DateStyle	ISO, MDY	client
  default_text_search_config	pg_catalog.english	configuration file
  dynamic_shared_memory_type	windows	configuration file
  extra_float_digits	3	session
  lc_messages	English_United States.1252	configuration file
  lc_monetary	English_United States.1252	configuration file
  lc_numeric	English_United States.1252	configuration file
  lc_time	English_United States.1252	configuration file
  listen_addresses	*	configuration file
  log_destination	stderr	configuration file
  logging_collector	on	configuration file
  max_connections	100	configuration file
  max_stack_depth	2MB	environment variable
  port	5432	configuration file
  shared_buffers	128MB	configuration file
  TimeZone	UTC	client

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