Обсуждение: create a temp table in SPI
I want to create some temporary tables in SPI, but after I created the table and inserted the data, I can’t query any data, why?
the postgres version:
PostgreSQL 13.6 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 7.3.0, 64-bit
code:
int ret = 0;
SPI_connect();
ret = SPI_execute("CREATE GLOBAL TEMP TABLE temp_table (id int, value text)", false, 0);
if(ret != SPI_OK_UTILITY)
{
elog(ERROR, "%s: create failed", __FUNCTION__);
}
ret = SPI_execute("INSERT INTO temp_table VALUES (1, 'a'), (2, 'b')", false, 0);
if(ret != SPI_OK_INSERT)
{
elog(ERROR, "%s: insert failed", __FUNCTION__);
}
// SPI_commit();
ret = SPI_execute("SELECT * FROM temp_table", true, 0);
if(ret != SPI_OK_SELECT)
{
elog(ERROR, "%s: select failed", __FUNCTION__);
}
if(SPI_processed > 0)
{
}
else {
// elog(ERROR, "%s: find nothing", __FUNCTION__);
}
// SPITupleTable *tuptable = SPI_tuptable;
// TupleDesc tupdesc = tuptable->tupdesc;
// // the numvals is 0,but I insert two records
// if (tuptable->numvals > 0)
// {
// HeapTuple tuple = tuptable->vals[0];
// char *tupleval1 = SPI_getvalue(tuple, tupdesc, 1);
// char *tupleval2 = SPI_getvalue(tuple, tupdesc, 2);
// pfree(tupleval1);
// pfree(tupleval2);
// }
// else
// {
// elog(ERROR, "%s: can not query something", __FUNCTION__);
// }
// do something with tuptable
SPI_finish();
On Thu, 2023-07-13 at 13:12 +0800, 黄宁 wrote:
> I want to create some temporary tables in SPI, but after I created the table and inserted the data, I can’t query any
data,why?
>
> the postgres version:
> PostgreSQL 13.6 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 7.3.0, 64-bit
> code:
>
>
> int ret = 0;
> SPI_connect();
> ret = SPI_execute("CREATE GLOBAL TEMP TABLE temp_table (id int, value text)", false, 0);
>
> ret = SPI_execute("INSERT INTO temp_table VALUES (1, 'a'), (2, 'b')", false, 0);
>
> ret = SPI_execute("SELECT * FROM temp_table", true, 0);
>
> if(SPI_processed > 0)
...
That's because you set "read_only" to "true" when you executed the query,
so that the command counter is not incremented, and the query cannot see
the results from the previous statement.
The documentation is quite clear here:
It is generally unwise to mix read-only and read-write commands within
a single function using SPI; that could result in very confusing behavior,
since the read-only queries would not see the results of any database
updates done by the read-write queries.
Yours,
Laurenz Albe
Thanks for your explanation.
Laurenz Albe <laurenz.albe@cybertec.at> 于2023年7月13日周四 17:48写道：
On Thu, 2023-07-13 at 13:12 +0800, 黄宁 wrote:
> I want to create some temporary tables in SPI, but after I created the table and inserted the data, I can’t query any data, why?
>
> the postgres version:
> PostgreSQL 13.6 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 7.3.0, 64-bit
> code:
>
>
> int ret = 0;
> SPI_connect();
> ret = SPI_execute("CREATE GLOBAL TEMP TABLE temp_table (id int, value text)", false, 0);
>
> ret = SPI_execute("INSERT INTO temp_table VALUES (1, 'a'), (2, 'b')", false, 0);
>
> ret = SPI_execute("SELECT * FROM temp_table", true, 0);
>
> if(SPI_processed > 0)
...
That's because you set "read_only" to "true" when you executed the query,
so that the command counter is not incremented, and the query cannot see
the results from the previous statement.
The documentation is quite clear here:
It is generally unwise to mix read-only and read-write commands within
a single function using SPI; that could result in very confusing behavior,
since the read-only queries would not see the results of any database
updates done by the read-write queries.
Yours,
Laurenz Albe