Mysql – Query perfromance debugging

I have recently installed a new module(paypal express) in Oxid 6.0.x backend.
After installation, the backend menu point "orders", in my backend, is timing out. Cause the query needs too long. I am not able to view orders (which is a CMS vendor resource).

I already tried to debug it with our managed hoster support who doubled our ram to increased mysql buffer size as well as php timouts etc. within the php.ini.

After the hoster php.ini change I still see the query with status "Copying to tmp table" even so the hoster has increased the query buffers by more then double

Question:
I am really not sure what the problem is. Does someone have an Idea what else I might try?
I cannot imagine that this one small join needs so much buffer space and time to excecute. Especially since the join to payppaypalpluspayment is the same as to oxv_oxpayments_de which is instant and on the same primary key.

This is the query I see running in processlist
Showing rows 0 – 0 (1 total, Query took 200.9683 seconds.) Has already decreased from about 10 min after the php.ini change

select count(*) 
from  `oxorder`
LEFT JOIN `oxv_oxpayments_de` AS `payments` 
    on `payments`.oxid=oxorder.oxpaymenttype
LEFT JOIN `oxv_oxpayments_de` AS pluspayments 
    ON pluspayments.oxid = oxorder.oxpaymenttype
LEFT JOIN payppaypalpluspayment 
    ON payppaypalpluspayment.OXORDERID = oxorder.OXID
LEFT JOIN payppaypalpluspui 
    ON payppaypalpluspui.OXPAYMENTID = payppaypalpluspayment.OXPAYMENTID
where 1  and ( oxorder.oxfolder = 'ORDERFOLDER_NEW' )

count(*) Result 150000
The problem may be this join, the others are instant results
rows 0 – 0 (1 total, Query took 153.2391 seconds.)

Select count(*) from  `oxorder`
LEFT JOIN payppaypalpluspayment 
    ON payppaypalpluspayment.OXORDERID = oxorder.OXID

EXPLAIN

id select_type  table                   type    possible_keys   key         key_len     ref     rows        Extra   
1   SIMPLE      oxorder                 index       NULL        MAINIDX     10          NULL    146861      Using index
1   SIMPLE      payppaypalpluspayment   index       NULL        OXORDERID   32          NULL    2630        Using where; Using index; Using join buffer (flat, BNL join)
1   SIMPLE      payppaypalpluspui       ALL         NULL        NULL        NULL        NULL    519         Using where

This explain changed after adding indexes in

oxorder.OXPAYMENTTYPE;
oxorder.oxfolder;
payppaypalpluspui.OXPAYMENTID;
payppaypalpluspayment.OXPAYMENTID;

id  select_type     table                   type    possible_keys   key         key_len ref                                 rows    Extra   
1   SIMPLE          oxorder                 ref     OXFOLDER        OXFOLDER    98      const                               73450   Using index condition
1   SIMPLE          payppaypalpluspayment   ALL     NULL            NULL        NULL    NULL                                2634    Using where; Using join buffer (flat, BNL join)
1   SIMPLE          payppaypalpluspui       ref     OXPAYMENTID     OXPAYMENTID 34      payppaypalpluspayment.OXPAYMENTID   1       Using where; Using index

Which seems more strange even so its a litle faster (still 170s)

After this count selects the backend is doing the same query with select oxorder.* and one column from payppaypalpluspui (takes about 600 seconds)
The UI has then already timed out. When I delete the last two joins and try it manually on the server the query takes < 1sec

dbs

CREATE TABLE `payppaypalpluspayment` (
 `OXID` char(32) CHARACTER SET latin1 COLLATE latin1_general_ci NOT NULL COMMENT 'Payment oxid id',
 `OXORDERID` char(32) CHARACTER SET latin1 COLLATE latin1_general_ci NOT NULL COMMENT 'Order id',
 `OXSALEID` varchar(32) CHARACTER SET latin1 COLLATE latin1_general_ci NOT NULL DEFAULT '' COMMENT 'PayPal Plus payment sale id',
 `OXPAYMENTID` varchar(32) CHARACTER SET latin1 COLLATE latin1_general_ci NOT NULL DEFAULT '' COMMENT 'PayPal Plus payment id',
 `OXSTATUS` varchar(32) NOT NULL DEFAULT '' COMMENT 'PayPal Plus payment status',
 `OXDATECREATED` datetime NOT NULL DEFAULT '0000-00-00 00:00:00' COMMENT 'Payment creation date',
 `OXTOTAL` double NOT NULL DEFAULT '0' COMMENT 'Total payment amount',
 `OXCURRENCY` varchar(32) NOT NULL DEFAULT '' COMMENT 'Payment currency',
 `OXPAYMENTOBJECT` blob NOT NULL COMMENT 'Serialized payment object',
 PRIMARY KEY (`OXID`),
 UNIQUE KEY `OXORDERID` (`OXORDERID`),
 UNIQUE KEY `OXSALEID` (`OXSALEID`)
 KEY `OXPAYMENTID` (`OXPAYMENTID`)    <<< added this index
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COMMENT='PayPal Plus payment data model'

Rows 3,228 InnoDB utf8_general_ci 11.4 MiB

CREATE TABLE `oxorder` (
 `OXID` char(32) NOT NULL COMMENT 'Order id',
 `OXSHOPID` int(11) NOT NULL DEFAULT '1' COMMENT 'Shop id (oxshops)',
 `OXUSERID` char(32) NOT NULL DEFAULT '' COMMENT 'User id (oxuser)',
 `OXORDERDATE` datetime NOT NULL DEFAULT '0000-00-00 00:00:00' COMMENT 'Order date',
 `OXORDERNR` varchar(16) NOT NULL COMMENT 'Order number',
 .....
 PRIMARY KEY (`OXID`),
 KEY `MAINIDX` (`OXSHOPID`,`OXSTORNO`,`OXORDERDATE`),
 KEY `OXORDERNR` (`OXORDERNR`)
 KEY `OXPAYMENTTYPE` (`OXPAYMENTTYPE`),  <<<< added this index
 KEY `OXFOLDER` (`OXFOLDER`)             <<<< added this index
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COMMENT='Shop orders information'

149,068 InnoDB utf8_general_ci 258.1 MiB

CREATE TABLE `payppaypalpluspui` (
 `OXID` char(32) CHARACTER SET latin1 COLLATE latin1_general_ci NOT NULL COMMENT 'Payment oxid id',
 `OXPAYMENTID` varchar(32) CHARACTER SET latin1 COLLATE latin1_general_ci NOT NULL DEFAULT '' COMMENT 'PayPal Plus payment id',
 `OXREFERENCENUMBER` varchar(255) NOT NULL DEFAULT '' COMMENT 'PayPal Plus PuI reference_number',
 `OXBANKNAME` varchar(255) NOT NULL DEFAULT '' COMMENT 'PayPal Plus PuI banking instruction bank name',
 `OXACCOUNTHOLDER` varchar(255) NOT NULL DEFAULT '' COMMENT 'PayPal Plus PuI banking instruction account holder',
 `OXIBAN` varchar(255) NOT NULL DEFAULT '' COMMENT 'PayPal Plus PuI banking instruction IBAN',
 `OXBIC` varchar(255) NOT NULL DEFAULT '' COMMENT 'PayPal Plus PuI banking instruction BIC',
 `OXDUEDATE` datetime NOT NULL DEFAULT '0000-00-00 00:00:00' COMMENT 'PayPal Plus PuI due date',
 `OXTOTAL` double NOT NULL DEFAULT '0' COMMENT 'PayPal Plus PuI Total invoice amount',
 `OXCURRENCY` varchar(32) NOT NULL DEFAULT '' COMMENT 'PayPal Plus PuI invoice currency',
 `OXPUIOBJECT` text NOT NULL COMMENT 'JSON representation of the payment instructions',
 PRIMARY KEY (`OXID`)
 KEY `OXPAYMENTID` (`OXPAYMENTID`)   <<<< added this index
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COMMENT='PayPal Plus Pay upon Invoice data model'

Rows 655 InnoDB utf8_general_ci 1.5 MiB

innodb buffer pool = 8 GiB
RAM 10 GiB