Add SQL tuning section

README.md

@@ -1015,3 +1015,53 @@ In most systems, reads can heavily number writes 100:1 or even 1000:1.  A read r
 ###### Source(s) and further reading: denormalization
 
 * [Denormalization](https://en.wikipedia.org/wiki/Denormalization)
+
+#### SQL tuning
+
+SQL tuning is a broad topic and many [books](https://www.amazon.com/s/ref=nb_sb_noss_2?url=search-alias%3Daps&field-keywords=sql+tuning) have been written as reference.
+
+It's important to **benchmark** and **profile** to simulate and uncover bottlenecks.
+
+* **Benchmark** - Simulate high-load situations with tools such as [ab](http://httpd.apache.org/docs/2.2/programs/ab.html).
+* **Profile** - Enable tools such as the [slow query log](http://dev.mysql.com/doc/refman/5.7/en/slow-query-log.html) to help track performance issues.
+
+Benchmarking and profiling might point you to the following optimizations.
+
+##### Tighten up the schema
+
+* MySQL dumps to disk in contiguous blocks for fast access.
+* Use `CHAR` instead of `VARCHAR` for fixed-length fields.
+    * `CHAR` effectively allows for fast, random access, whereas with `VARCHAR`, you must find the end of a string before moving onto the next one.
+* Use `TEXT` for large blocks of text such as blog posts.  `TEXT` also allows for boolean searches.  Using a `TEXT` field results in storing a pointer on disk that is used to locate the text block.
+* Use `INT` for larger numbers up to 2^32 or 4 billion.
+* Use `DECIMAL` for currency to avoid floating point representation errors.
+* Avoid storing large `BLOBS`, store the location of where to get the object instead.
+* `VARCHAR(255)` is the largest number of characters that can be counted in an 8 bit number, often maximizing the use of a byte in some RDBMS.
+* Set the `NOT NULL` constraint where applicable to [improve search performance](http://stackoverflow.com/questions/1017239/how-do-null-values-affect-performance-in-a-database-search).
+
+##### Use good indices
+
+* Columns that you are querying (`SELECT`, `GROUP BY`, `ORDER BY`, `JOIN`) could be faster with indices.
+* Indices are usually represented as self-balancing [B-tree](https://en.wikipedia.org/wiki/B-tree) that keeps data sorted and allows searches, sequential access, insertions, and deletions in logarithmic time.
+* Placing an index can keep the data in memory, requiring more space.
+* Writes could also be slower since the index also needs to be updated.
+* When loading large amounts of data, it might be faster to disable indices, load the data, then rebuild the indices.
+
+##### Avoid expensive joins
+
+* [Denormalize](#denormalization) where performance demands it.
+
+##### Partition tables
+
+* Break up a table by putting hot spots in a separate table to help keep it in memory.
+
+##### Tune the query cache
+
+* In some cases, the [query cache](http://dev.mysql.com/doc/refman/5.7/en/query-cache) could lead to [performance issues](https://www.percona.com/blog/2014/01/28/10-mysql-performance-tuning-settings-after-installation/).
+
+##### Source(s) and further reading: SQL tuning
+
+* [Tips for optimizing MySQL queries](http://20bits.com/article/10-tips-for-optimizing-mysql-queries-that-dont-suck)
+* [Is there a good reason i see VARCHAR(255) used so often?](http://stackoverflow.com/questions/1217466/is-there-a-good-reason-i-see-varchar255-used-so-often-as-opposed-to-another-l)
+* [How do null values affect performance?](http://stackoverflow.com/questions/1017239/how-do-null-values-affect-performance-in-a-database-search)
+* [Slow query log](http://dev.mysql.com/doc/refman/5.7/en/slow-query-log.html)