BUG #15614: Query plan: buffer stats from workers in child operations discarded.

The following bug has been logged on the website:

Bug reference: 15614
Logged by: Dave Conlin
Email address: dc345(at)cantab(dot)net
PostgreSQL version: 10.4
Operating system: Windows 10
Description:

Hi folks -

In the output of an `EXPLAIN` query, normally child operations are included
in the totals for the buffers values, eg "Shared Hit Blocks". It seems that
when a parent operation is single-threaded, but a child operation starts
workers, then only the buffers stats from the main thread in the child
operation are included in the totals for the parent operation.

A worked example:

I've run the following query. The "towns" table is a massive table that I
created for testing purposes - don't worry, this isn't a real query, just a
silly example to get some big numbers on a parallel query.

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
}
]
```

The Sort operation has a "Shared Read Blocks" value of 142,762, with a
breakdown of:
46,794 in worker 0
48,327 in worker 1
47,641 in the main thread.

Its parent, the Gather Merge operation, has a "Shared Read Blocks" value of
47,641. This clearly doesn't include all the total blocks from its children
(it's less than the number for its only child), but it is equal to the value
for the main thread for the Sort operation. This would make sense if worker
values are being discarded/lost, since I wouldn't expect the gather merge to
be reading any new data.

This pattern is identical for all 10 of the buffers values - the value for
the Gather Merge operation is equal to the value for the main thread in the
Sort child operation.

So it looks like the buffers values for worker threads are discarded for
operations which don't manage the workers - is this intended/desired
behaviour?

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

Dave