Re: pgbench -f and vacuum

From: Tomas Vondra <tv(at)fuzzy(dot)cz>
To: pgsql-hackers(at)postgresql(dot)org
Subject: Re: pgbench -f and vacuum
Date: 2014-12-21 23:28:46
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On 21.12.2014 15:58, Tatsuo Ishii wrote:
>> On Sun, Dec 14, 2014 at 11:43 AM, Tatsuo Ishii <ishii(at)postgresql(dot)org> wrote:
>>>> If we care enough about that case to attempt the vacuum anyway
>>>> then we need to do something about the error message; either
>>>> squelch it or check for the existence of the tables before
>>>> attempting to vacuum. Since there's no way to squelch in the
>>>> server logfile, I think checking for the table is the right
>>>> answer.
>>> Fair enough. I will come up with "checking for table before
>>> vacuum" approach.
>> +1 for this approach.
> Here is the patch I promised.

First of all - I'm not entirely convinced the "IF EXISTS" approach is
somehow better than "-f implies -n" suggested before, but I don't have a
strong preference either.

Regarding the patch:

(1) I agree with Fabrizio that the 'executeStatement2' is not the best
naming as it does not show the 'if exists' intent.

(2) The 'executeStatement2' API is a bit awkward as the signarure

executeStatement2(PGconn *con, const char *sql, const char *table);

suggests that the 'sql' command is executed when 'table' exists.
But that's not the case, because what actually gets executed is
'sql table'.

(3) The 'is_table_exists' should be probably just 'table_exists'.

(4) The SQL used in is_table_exists to check table existence seems
slightly wrong, because 'to_regclass' works for other relation
kinds, not just regular tables - it will match views for example.
While a conflict like that (having an object with the right name
but not a regular table) is rather unlikely I guess, a more correct
query would be this:

SELECT oid FROM pg_class WHERE relname = '%s' AND relkind = 'r';

(5) I'm not a libpq expert, but I don't see how the PQgetvalue() could
return anything except true/false, so the

if (result == NULL)
return false;

seems a bit pointless to me. But maybe it's necessary?

(6) The is_table_exists might be further simplified along these lines:

static bool
is_table_exists(PGconn *con, const char *table)
PGresult *res;
char buf[1024];
char *result;
bool retval;

snprintf(buf, sizeof(buf)-1,
"SELECT to_regclass('%s') IS NULL", table);

res = PQexec(con, buf);
if (PQresultStatus(res) != PGRES_TUPLES_OK)
return false;

result = PQgetvalue(res, 0, 0);

retval = (*result == 't');


return retval;

(7) I also find the coding in main() around line 3250 a bit clumsy. The
first reason is that it only checks existence of 'pgbench_branches'
and then vacuums three pgbench_tellers and pgbench_history in the
same block. If pgbench_branches does not exist, there will be no
message printed (but the tables will be vacuumed).

The second reason is that the msg1, msg2 variables seem unnecessary.
IMHO this is a bit better:

if (!is_no_vacuum)
if (is_table_exists(con, "pgbench_branches"))
fprintf(stderr, "starting vacuum...");

executeStatement2(con, "vacuum", "pgbench_branches");
executeStatement2(con, "vacuum", "pgbench_tellers");
executeStatement2(con, "truncate", "pgbench_history");

fprintf(stderr, "end.\n");

if (do_vacuum_accounts)
if (is_table_exists(con, "pgbench_accounts"))
fprintf(stderr, "starting vacuum pgbench_accounts...");

"vacuum analyze pgbench_accounts");

fprintf(stderr, "end.\n");

(8) Also, I think it's not necessary to define function prototypes for
executeStatement2 and is_table_exists. It certainly is not
consistent with the other functions defined in pgbench.c (e.g.
there's no prototype for executeStatement). Just delete the two
prototypes and move is_table_exists before executeStatement2.


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