I'm looking for best practice on how to query to database in observer.
Ok so this is the idea, i use observer to catch event catalog_product_save_after (which will be fired after product is saved in admin page), after that, i get product id, and query to database to get data. Then after some code, i will update data to database
I've searched a lot, but what i found is just some direct sql solution (which i don't think is good practice for this). Here is what i've found:
Magento 2 direct SQL query for insert not working
magento 2 , write custom query in customer_address_entity, We can't save the address
Magento 2: join custom table with product collection while sql query is running fine
(i was not try the third one)
Ok so here is what i do so far:

events.xml

<?xml version="1.0"?>
<!--
/**
 * Copyright © 2013-2017 Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
-->
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
    <event name="sales_model_service_quote_submit_success">
        <observer name="sendEmailCustom" instance="Synergy\Butility\Observer\Quote\Webapi\SubmitObserver" />
    </event>
     <event name="catalog_product_save_after">
        <observer name="saveProduct" instance="Synergy\Butility\Observer\ProductSaveObserver" />
    </event>
</config>

ProductSaveObserver.php

<?php

namespace Synergy\Butility\Observer;

use Magento\Framework\Event\ObserverInterface;

class ProductSaveObserver implements ObserverInterface
{    
    public function __construct(
        \Magento\Framework\App\ResourceConnection $resource
    ) {
        $this->_resource = $resource;
    }

    public function execute(\Magento\Framework\Event\Observer $observer)
    {
        $productId = $observer->getProduct()->getId();

        $objectManager = \Magento\Framework\App\ObjectManager::getInstance();
        $resource = $objectManager->get('\Magento\Framework\App\ResourceConnection');
        $connection = $resource->getConnection();
        $tableName = $resource->getTableName('catalog_product_flat_32');

        $product_query = "SELECT * FROM " . $tableName . " WHERE entity_id = " . $productId;

        $product = $connection->query($product_query);
        foreach ($product as $data) {
            var_dump($product->getData());
        }
        exit;
        // $sql = "UPDATE " . $tableName . " SET customer_type = '" . $this->getCustomerType() . "'  WHERE entity_id = " . $this->getEntityId($observer);
        // $connection->query($sql);
    }   
}

Thanks for reading :)