When use prepared protocol, transaction will hold backend_xmin until the end of the transaction.

[chenhj]

Hi, hackers!

When execute sql with prepared protocol, read committed transaction will hold 
backend_xmin until the end of the transaction.

Is this behavior normal?

Should read committed transaction release backend_xmin immediately after SQL 
executing is completed? Just like
when executing sql with simple protocol.

# reproduction
## env
- PostgreSQL 9.2
- CentOS 7.2

## test script

    $ cat test.sql
    begin;
    select 1;
    \sleep 1000s


## execute with simple protocol

    $ pgbench -n -t 1 -f test.sql "service=admin"

    postgres=# select * from pg_stat_activity where query='select 1;';
    -[ RECORD 1 ]----+------------------------------
    datid            | 13805
    datname          | postgres
    pid              | 19641
    usesysid         | 16388
    usename          | admin
    application_name | pgbench
    client_addr      |
    client_hostname  |
    client_port      | -1
    backend_start    | 2018-07-04 13:27:10.62635+08
    xact_start       | 2018-07-04 13:27:10.629609+08
    query_start      | 2018-07-04 13:27:10.629845+08
    state_change     | 2018-07-04 13:27:10.63035+08
    wait_event_type  | Client
    wait_event       | ClientRead
    state            | idle in transaction
    backend_xid      |
    backend_xmin     |
    query            | select 1;
    backend_type     | client backend

## execute with prepared protocol

    $ pgbench -n -t 1 -f test.sql "service=admin" -M prepared

    postgres=# select * from pg_stat_activity where query='select 1;';
    -[ RECORD 1 ]----+------------------------------
    datid            | 13805
    datname          | postgres
    pid              | 19662
    usesysid         | 16388
    usename          | admin
    application_name | pgbench
    client_addr      |
    client_hostname  |
    client_port      | -1
    backend_start    | 2018-07-04 13:27:46.637134+08
    xact_start       | 2018-07-04 13:27:46.641348+08
    query_start      | 2018-07-04 13:27:46.64174+08
    state_change     | 2018-07-04 13:27:46.641778+08
    wait_event_type  | Client
    wait_event       | ClientRead
    state            | idle in transaction
    backend_xid      |
    backend_xmin     | 3930269815
    query            | select 1;
    backend_type     | client backend

backend_xmin will affect dead tuple removing

    postgres=# create table tbchj(id int);
    CREATE TABLE
    postgres=# insert into tbchj values(1);
    INSERT 0 1
    postgres=# delete from tbchj;
    DELETE 1
    postgres=# vacuum VERBOSE tbchj;
    INFO:  vacuuming "public.tbchj"
    INFO:  "tbchj": found 0 removable, 1 nonremovable row versions in 1 out of 
1 pages
    DETAIL:  1 dead row versions cannot be removed yet, oldest xmin: 3930269815
    There were 0 unused item pointers.
    Skipped 0 pages due to buffer pins, 0 frozen pages.
    0 pages are entirely empty.
    CPU: user: 0.00 s, system: 0.00 s, elapsed: 0.00 s.
    VACUUM
 
Regards
Chen Huajun