pg_field_type first call is very slow

[vicreal]

Description

Problem: function pg_field_type first call (after run php script) is very slow.
The following code:

$connection = pg_connect("...") or die('Cannot connect db');
$res = pg_query($connection, 'SELECT 100');

$time0 = microtime(true); pg_field_name($res, 0);
$out[] = '(query 1) pg_field_name() first call = '.number_format((microtime(true) - $time0) * 1000, 4).' мс';

$time0 = microtime(true); pg_field_size($res, 0);
$out[] = '(query 1) pg_field_size() first call = '.number_format((microtime(true) - $time0) * 1000, 4).' мс';

$time0 = microtime(true); pg_field_type($res, 0);
$out[] = '(query 1) pg_field_type() first call = '.number_format((microtime(true) - $time0) * 1000, 4).' мс';

$time0 = microtime(true); pg_field_type($res, 0);
$out[] = '(query 1) pg_field_type() second call = '.number_format((microtime(true) - $time0) * 1000, 4).' мс';

$res = pg_query($connection, 'SELECT 200');

$time0 = microtime(true); pg_field_type($res, 0);
$out[] = '(query 2) pg_field_type() first call = '.number_format((microtime(true) - $time0) * 1000, 4).' мс';

$time0 = microtime(true); pg_field_type($res, 0);
$out[] = '(query 2) pg_field_type() second call = '.number_format((microtime(true) - $time0) * 1000, 4).' мс';

print_r($out);
<?php

Resulted in this output:

Array (
    [0] => "(query 1) pg_field_name() first call = 0.0069 мс"
    [1] => "(query 1) pg_field_size() first call = 0.0031 мс"
    [2] => "(query 1) pg_field_type() first call = 46.6821 мс"  // FIRST CALL IS VERY SLOW (pg_field_type)
    [3] => "(query 1) pg_field_type() second call = 0.0050 мс"
    [4] => "(query 2) pg_field_type() first call = 0.0069 мс"
    [5] => "(query 2) pg_field_type() second call = 0.0029 мс"
)

But I expected this output instead:

Array (
    [0] => "(query 1) pg_field_name() first call = 0.0069 мс"
    [1] => "(query 1) pg_field_size() first call = 0.0031 мс"
    [2] => "(query 1) pg_field_type() first call = 0.0050-0.0069 мс"  // MUST BE
    [3] => "(query 1) pg_field_type() second call = 0.0050 мс"
    [4] => "(query 2) pg_field_type() first call = 0.0069 мс"
    [5] => "(query 2) pg_field_type() second call = 0.0029 мс"
)

PHP Version

PHP 8.1

Operating System

Debian 11 (bullseye)

[cmb69]

IIRC, pg_field_type() needs to request the types from the DB server once, and caches them. That would explain the slowness of the first call.

[vicreal]

IIRC, pg_field_type() needs to request the types from the DB server once, and caches them. That would explain the slowness of the first call.

But if several different SQL queries are executed in a row, then the slowness run are observed only when the function is called for the first time after the first query. After the second SQL query, the function is executed quickly.

[vicreal]

When using a permanent connection (pg_pconnect), the problem is still observed: on each http-request first call of pg_field_type() function is very slow.

[cmb69]

Well, the problem is indeed that query:

php-src/ext/pgsql/pgsql.c

Line 1557 in 296a095

PGresult *result = PQexec(pgsql, "select oid,typname from pg_type");

See also https://bugs.php.net/bug.php?id=75705.

The info is cached in a module global:

php-src/ext/pgsql/pgsql.c

Line 1583 in 296a095

zend_hash_index_update_ptr(&PGG(field_oids), tmp_oid, name);

Module globals need to be initialized for all processes (or threads for ZTS builds), regardless of whether there is persistent or a regular connection.