Re: display hot standby state in psql prompt

On 30/10/2025 11:16, Jim Jones wrote:
> if (!hs || !ro)
> strlcpy(buf, _("unknown"), sizeof(buf));
> else if (strcmp(hs, "on") == 0 || strcmp(ro, "on") == 0)
> strlcpy(buf, _("read-only"), sizeof(buf));
> else
> {
> const char *tr = NULL;
> PGresult *res;
>
> res = PQexec(pset.db, "SHOW transaction_read_only");
> if (PQresultStatus(res) == PGRES_TUPLES_OK &&
> PQntuples(res) == 1)
> tr = PQgetvalue(res, 0, 0);
>
> if (!tr)
> strlcpy(buf, _("unknown"), sizeof(buf));
> else if (strcmp(tr, "on") == 0)
> strlcpy(buf, _("read-only"), sizeof(buf));
> else
> strlcpy(buf, _("read/write"), sizeof(buf));
>
> PQclear(res);
> }

While reviewing another patch, I had another idea to further minimise
the overhead of checking transaction_read_only, namely, to check it only
when within a transaction block:

if (!hs || !ro)
strlcpy(buf, _("unknown"), sizeof(buf));
else if (strcmp(hs, "on") == 0 || strcmp(ro, "on") == 0)
strlcpy(buf, _("read-only"), sizeof(buf));
else
{
PGTransactionStatusType tstatus = PQtransactionStatus(pset.db);

/*
* Check transaction_read_only only when in a transaction
* block. When idle (not in a transaction), the value of
* transaction_read_only is the same as
* default_transaction_read_only, which we already checked
* above. Avoiding the query improves performance,
* especially for prompt redisplays.
*/

if (tstatus == PQTRANS_IDLE)
strlcpy(buf, _("read/write"), sizeof(buf));
else
{
/* In a transaction block, need to check transaction_read_only */
const char *tr = NULL;
PGresult *res;

res = PQexec(pset.db, "SHOW transaction_read_only");
if (PQresultStatus(res) == PGRES_TUPLES_OK && PQntuples(res) == 1)
tr = PQgetvalue(res, 0, 0);

if (!tr)
strlcpy(buf, _("unknown"), sizeof(buf));
else if (strcmp(tr, "on") == 0)
strlcpy(buf, _("read-only"), sizeof(buf));
else
strlcpy(buf, _("read/write"), sizeof(buf));

PQclear(res);
}
}

The idea is to skip the test if tstatus == PQTRANS_IDLE, which indicates
that it is not in a transaction block, making the check for
transaction_read_only unnecessary.

Thoughts on this approach?

v7 attached.

Best, Jim