feat: add explain mysql

docs/2022-07-31-sql.html

@@ -131,6 +131,23 @@
       <pre><span class="pl-k">CREATE</span> <span class="pl-k">INDEX</span> `<span class="pl-en">idx_timestamp</span>`
     <span class="pl-k">ON</span> <span class="pl-s"><span class="pl-pds">`</span>user_upload<span class="pl-pds">`</span></span> (<span class="pl-s"><span class="pl-pds">`</span>timestamp<span class="pl-pds">`</span></span>);</pre>
     </div>
+    <p>Use <code>EXPLAIN</code> to check if index is used or not:</p>
+    <ul>
+      <li>
+        <a
+          href="https://dev.mysql.com/doc/refman/5.7/en/explain-output.html"
+          rel="nofollow"
+          >For MySQL 5.7</a
+        >
+      </li>
+      <li>
+        <a
+          href="https://dev.mysql.com/doc/refman/8.0/en/explain-output.html"
+          rel="nofollow"
+          >For MySQL 8.0</a
+        >
+      </li>
+    </ul>
     <h2>
       <a
         id="user-content-be-careful-with-null"
@@ -273,6 +290,13 @@
           >My Notes on GitLab Postgres Schema Design</a
         >
       </li>
+      <li>
+        <a
+          href="https://planetscale.com/blog/how-read-mysql-explains"
+          rel="nofollow"
+          >How to read MySQL EXPLAINs</a
+        >
+      </li>
     </ul>
 
     <div>

posts/2022-07-31-sql.md

@@ -53,6 +53,11 @@ CREATE INDEX `idx_timestamp`
     ON `user_upload` (`timestamp`);
 ```
 
+Use `EXPLAIN` to check if index is used or not:
+
+- [For MySQL 5.7](https://dev.mysql.com/doc/refman/5.7/en/explain-output.html)
+- [For MySQL 8.0](https://dev.mysql.com/doc/refman/8.0/en/explain-output.html)
+
 ## Be careful with NULL
 
 If compare with field which can be NULL, remember to check NULL for safety.
@@ -97,3 +102,4 @@ Plase read docs about online ddl operations before do anything online (keep data
 - [Things You Should Know About Databases](https://architecturenotes.co/things-you-should-know-about-databases/)
 - [When to use JSON data type in database schema design?](https://shekhargulati.com/2022/01/08/when-to-use-json-data-type-in-database-schema-design/)
 - [My Notes on GitLab Postgres Schema Design](https://shekhargulati.com/2022/07/08/my-notes-on-gitlabs-postgres-schema-design/)
+- [How to read MySQL EXPLAINs](https://planetscale.com/blog/how-read-mysql-explains)