'SHOW TABLES'(used in function createTempTables) is a MySQLism, which is crashing PostgreSQL sites

Problem/Motivation

After creating a new Drupal 8.9 site, and adding Cleantalk, I find that the site is WSODing with the error:

Drupal\Core\Database\DatabaseExceptionWrapper: SQLSTATE[42601]: Syntax error: 7 ERROR: syntax error at or near "::" LINE 1: SHOW TABLES::text LIKE 'cleantalk_sfw' ^: SHOW TABLES LIKE 'cleantalk_sfw'; Array ( ) in Cleantalk\ApbctDrupal\DB->fetch() (line 62 of /home/accdesign/public_html/web/modules/contrib/cleantalk/src/lib/Cleantalk/ApbctDrupal/DB.php)
#0 /home/accdesign/public_html/web/core/lib/Drupal/Core/Database/Connection.php(665): Drupal\Core\Database\Connection->handleQueryException()
#1 /home/accdesign/public_html/web/core/lib/Drupal/Core/Database/Driver/pgsql/Connection.php(202): Drupal\Core\Database\Connection->query()
#2 /home/accdesign/public_html/web/modules/contrib/cleantalk/src/lib/Cleantalk/ApbctDrupal/DB.php(62): Drupal\Core\Database\Driver\pgsql\Connection->query()
#3 /home/accdesign/public_html/web/modules/contrib/cleantalk/src/lib/Cleantalk/Common/Firewall/FirewallUpdater.php(379): Cleantalk\ApbctDrupal\DB->fetch()
#4 /home/accdesign/public_html/web/modules/contrib/cleantalk/src/lib/Cleantalk/Common/Firewall/FirewallUpdater.php(131): Cleantalk\Common\Firewall\FirewallUpdater->createTempTables()
#5 /home/accdesign/public_html/web/modules/contrib/cleantalk/src/CleantalkFuncs.php(675): Cleantalk\Common\Firewall\FirewallUpdater->update()
#6 /home/accdesign/public_html/web/modules/contrib/cleantalk/src/lib/Cleantalk/ApbctDrupal/RemoteCalls.php(13): Drupal\cleantalk\CleantalkFuncs::apbct_sfw_update()
#7 /home/accdesign/public_html/web/modules/contrib/cleantalk/src/lib/Cleantalk/Common/RemoteCalls.php(110): Cleantalk\ApbctDrupal\RemoteCalls->action__sfw_update()
#8 /home/accdesign/public_html/web/modules/contrib/cleantalk/src/EventSubscriber/BootSubscriber.php(136): Cleantalk\Common\RemoteCalls->perform()
#9 /home/accdesign/public_html/web/core/lib/Drupal/Core/StackMiddleware/ReverseProxyMiddleware.php(47): Drupal\cleantalk\EventSubscriber\BootSubscriber->handle()
#10 /home/accdesign/public_html/web/core/lib/Drupal/Core/StackMiddleware/NegotiationMiddleware.php(52): Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle()
#11 /home/accdesign/public_html/vendor/stack/builder/src/Stack/StackedHttpKernel.php(23): Drupal\Core\StackMiddleware\NegotiationMiddleware->handle()
#12 /home/accdesign/public_html/web/core/lib/Drupal/Core/DrupalKernel.php(708): Stack\StackedHttpKernel->handle()
#13 /home/accdesign/public_html/web/index.php(19): Drupal\Core\DrupalKernel->handle()
#14 {main}
.

I had reported a similar error awhile back, but didn't realize the real problem: 'SHOW TABLES' is a MYSQLism that isn't supported in PostgreSQL. To do the equivalent in PostgreSQL, you have to do a query against pg_catalog.pg_tables. See, for instance https://www.postgresqltutorial.com/postgresql-show-tables/ under the title "PostgreSQL show tables using pg_catalog schema"

Steps to reproduce

Install Cleantalk on a Drupal 8 or 9 site using PostgreSQL

Proposed resolution

The code needs to be changed to detect the use of PostgreSQL, and use an appropriate query against pg_catalog.pg_tables to test for the existence a table.

Workaround

Turn off SFW

Patch forthcoming

I can try to work up a patch on this in the next few days. If you'd rather handle it yourself, and don't have a PostgreSQL setup to test on, I'd be glad to test whatever you come up with.