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?
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
