It would be nice to clarify is there any point in select queries pipelining - Mailing list pgsql-docs

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/16/protocol-flow.html
Description:

Greeting!
Please consider the following exchange with a PG database (Kotlin + Reactor+
r2dbc-postgresql):
```
     295    1.063166    127.0.0.1   50591   127.0.0.1   32797   PGSQL   111
>Q ---> BEGIN ISOLATION LEVEL REPEATABLE READ, READ WRITE
     296    1.063219    127.0.0.1   32797   127.0.0.1   50591   TCP 56
32797 → 50591 [ACK] Seq=1 Ack=56 Win=6373 Len=0 TSval=3266177882
TSecr=3728690767
     301    1.069912    127.0.0.1   32797   127.0.0.1   50591   PGSQL   73
<C/Z
     302    1.069938    127.0.0.1   50591   127.0.0.1   32797   TCP 56
50591 → 32797 [ACK] Seq=56 Ack=18 Win=6370 Len=0 TSval=3728690774
TSecr=3266177889
     712    1.099829    127.0.0.1   50591   127.0.0.1   32797   PGSQL   551
>Q ------> select * from ... (#2)
     713    1.099858    127.0.0.1   32797   127.0.0.1   50591   TCP 56
32797 → 50591 [ACK] Seq=18 Ack=551 Win=6365 Len=0 TSval=3266177919
TSecr=3728690804
     715    1.099985    127.0.0.1   50591   127.0.0.1   32797   PGSQL   551
>Q ------> select * from ... (#4)
     717    1.100009    127.0.0.1   32797   127.0.0.1   50591   TCP 56
32797 → 50591 [ACK] Seq=18 Ack=1046 Win=6358 Len=0 TSval=3266177919
TSecr=3728690804
     719    1.100082    127.0.0.1   50591   127.0.0.1   32797   PGSQL   551
>Q ------> select * from ... (#1)
     720    1.100106    127.0.0.1   32797   127.0.0.1   50591   TCP 56
32797 → 50591 [ACK] Seq=18 Ack=1541 Win=6350 Len=0 TSval=3266177919
TSecr=3728690804
     722    1.100164    127.0.0.1   50591   127.0.0.1   32797   PGSQL   551
>Q ------> select * from ... (#3)
     723    1.100192    127.0.0.1   32797   127.0.0.1   50591   TCP 56
32797 → 50591 [ACK] Seq=18 Ack=2036 Win=6342 Len=0 TSval=3266177919
TSecr=3728690804
     735    1.114695    127.0.0.1   32797   127.0.0.1   50591   PGSQL   424
<T/D/C/Z ------> Results for #2
     737    1.114741    127.0.0.1   50591   127.0.0.1   32797   TCP 56
50591 → 32797 [ACK] Seq=2036 Ack=386 Win=6364 Len=0 TSval=3728690818
TSecr=3266177933
     773    1.121732    127.0.0.1   32797   127.0.0.1   50591   PGSQL   1468
   <T/D/D/D/D/D/D/D/D/D/D/D/D/C/Z ------> Results for #4
     774    1.121757    127.0.0.1   50591   127.0.0.1   32797   TCP 56
50591 → 32797 [ACK] Seq=2036 Ack=1798 Win=6342 Len=0 TSval=3728690826
TSecr=3266177941
     785    1.126793    127.0.0.1   32797   127.0.0.1   50591   PGSQL   594
<T/D/D/D/C/Z ------> Results for #1
     786    1.126820    127.0.0.1   50591   127.0.0.1   32797   TCP 56
50591 → 32797 [ACK] Seq=2036 Ack=2336 Win=6334 Len=0 TSval=3728690831
TSecr=3266177946
     805    1.135197    127.0.0.1   32797   127.0.0.1   50591   PGSQL   497
<T/D/D/C/Z ------> Results for #3
     806    1.135222    127.0.0.1   50591   127.0.0.1   32797   TCP 56
50591 → 32797 [ACK] Seq=2036 Ack=2777 Win=6327 Len=0 TSval=3728690839
TSecr=3266177954
     847    1.138848    127.0.0.1   50591   127.0.0.1   32797   PGSQL   68
>Q ------> COMMIT
     848    1.138876    127.0.0.1   32797   127.0.0.1   50591   TCP 56
32797 → 50591 [ACK] Seq=2777 Ack=2048 Win=6342 Len=0 TSval=3266177958
TSecr=3728690843
     853    1.144624    127.0.0.1   32797   127.0.0.1   50591   PGSQL   74
<C/Z
```

The application code is the following (for the sake of clarity):
```
val transactionalOperator = TransactionalOperator.create(
    transactionManager,
    DefaultTransactionDefinition() .apply {
        isolationLevel = TransactionDefinition.ISOLATION_REPEATABLE_READ
    }
)

val users = Flux.defer {
    Flux.create {
        it.next(1)
        it.next(2)
        it.next(3)
        it.next(4)
        it.complete()
    }
        .parallel(4)
        .runOn(Schedulers.parallel())
        .flatMap {
            databaseClient.sql(
                """
                    select * from …
                """.trimIndent())
                .map { row, _ -> EntityConverter().convert(row) }
                .all()
                .reduce(EntityReducer())
        }
}.`as`(transactionalOperator::transactional)
```

I’m aware that Postgres doesn’t support cursor(portal) multiplexing so its
impossible to fetch data for two selects simultaneously but should I make
use of selects pipelining when I want to make the most of the DB? I mean to
say does a single Postgres backend employs concurrent processing and its
possible for a DB to transmit results for a query #2 (see the traffic) and
concurrently parse/rewrite/plan/optimize the subsequent (pipelined) queries
(issued from the same transaction). Neither chapter 55 nor 52 expand on it
(or it’s hard to spot at least).

Thanks!