Re: Why overhead of SPI is so large?

On 22.08.2019 3:27, Tsunakawa, Takayuki wrote:
> From: Konstantin Knizhnik [mailto:k(dot)knizhnik(at)postgrespro(dot)ru]
>> PL/pgSQL: 29044.361 ms
>> C/SPI: 22785.597 ms
>>
>> The fact that difference between PL/pgSQL and function implemented in C
>> using SPI is not so large was expected by me.
> This PL/pgSQL overhead is not so significant compared with the three times, but makes me desire some feature like Oracle's ALTER PROCEDURE ... COMPILE; that compiles the PL/SQL logic to native code. I've seen a few dozen percent speed up.
>

Actually my implementation of C/SPI version is not optimal: it is better
to fetch several records:

    while (true)
    {
        SPI_cursor_fetch(portal, true, 100);
        if (SPI_processed) {
            for (i = 0; i < SPI_processed; i++) {
                HeapTuple spi_tuple = SPI_tuptable->vals[i];
                Datum val = SPI_getbinval(spi_tuple,
SPI_tuptable->tupdesc, 1, &is_null);
                double x = DatumGetFloat8(val);
                result += x*x;
                SPI_freetuple(spi_tuple);
            }
            SPI_freetuptable(SPI_tuptable);
        } else
            break;
    }

This version shows result 9405.694 ms which is comparable with result of
SQL query.
Unfortunately (or fortunately) PL/pgSQL is already using prefetch. If it
is disables (when iterate through explicitly created cursor), time of
query execution is increased almost twice (46552.935 ms)

So PL/SPI ratio is more than three times.

Updatede results are the following:

Impementation
time (ms)
PL/Lua 32220
PL/pgSQL 29044
PL/pgSQL (JIT)
27594
C/SPI   9406
SQL   7399
SQL (JIT)
  5532
С/coreAPI   2873

--
> Konstantin Knizhnik
> Postgres Professional: http://www.postgrespro.com
> The Russian Postgres Company