Bulk INSERT...ON DUPLICATE KEY UPDATE updates only a fraction of rows

CREATE TABLE `demo_upsert` (

    `ID` BIGINT(20) NOT NULL AUTO_INCREMENT,

    `Url` TEXT NOT NULL COLLATE 'utf8mb4_unicode_ci',

    `LastListID` BIGINT(20) NULL DEFAULT NULL,

    `Statistics` BIGINT(20) NOT NULL DEFAULT '0',

    PRIMARY KEY (`ID`) USING BTREE,

    UNIQUE INDEX `Url` (`Url`) USING HASH

)

COLLATE='utf8mb4_unicode_ci'

ENGINE=InnoDB

;

#include <cassert>

#include <string>

#include <random>

#include <iostream>

#include "mariadb/mysql.h"

#include "Secret.h" // for database credentials

using namespace std;

string randStr(const int len, mt19937_64& rng) {

  string ans;

  for (int i = 0; i < len; i++) {

    ans += ('a' + rng() % 26);

  }

  return ans;

}

int main() {

  MYSQL mysql;

  mysql_library_init(0, nullptr, nullptr);

  mysql_thread_init();

  MYSQL* pmsRet = mysql_init(&mysql);

  assert(pmsRet);

  pmsRet = mysql_real_connect(&mysql, ConnProp::_cpHost, ConnProp::_cpUser, ConnProp::_cpPasswd,

    ConnProp::_cpDB, ConnProp::_cpPort, nullptr, CLIENT_COMPRESS | CLIENT_LOCAL_FILES | CLIENT_MULTI_STATEMENTS

    | CLIENT_MULTI_RESULTS);

  assert(pmsRet);

  const int lastListID = 10;

  mt19937_64 rng;

  for (int i = 0; i < 3; i++) {

    string query;

    query = "INSERT INTO demo_upsert (Url, LastListID) VALUES ";

    for (int j = 0; j < 15000; j++) {

      if (j != 0) {

        query += ", ";

      }

      query += "('" + randStr(3, rng) + "', " + to_string(lastListID) + ")";

    }

    query += " ON DUPLICATE KEY UPDATE LastListID=" + to_string(lastListID);

    int iRet = mysql_real_query(&mysql, query.c_str(), query.size());

    assert(iRet == 0);

    int64_t nAffected = mysql_affected_rows(&mysql);

    cout << nAffected << ' ';

    query = "UPDATE demo_upsert SET LastListID=NULL";

    iRet = mysql_real_query(&mysql, query.c_str(), query.size());

    assert(iRet == 0);

    nAffected = mysql_affected_rows(&mysql);

    cout << nAffected << endl;

  }

  return 0;

}