Add Relational Databases course (#21)

Looks good to me

courses/databases_sql/innodb.md

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+### Why should you use this?
+
+General purpose, row level locking, ACID support, transactions, crash recovery and multi-version concurrency control etc.
+
+
+### Architecture
+
+![alt_text](images/innodb_architecture.png "InnoDB components")
+
+
+### Key components:
+
+*   Memory:
+    *   Buffer pool: LRU cache of frequently used data(table and index) to be processed directly from memory, which speeds up processing. Important for tuning performance.
+    *   Change buffer: Caches changes to secondary index pages when those pages are not in the buffer pool and merges it when they are fetched. Merging may take a long time and impact live queries. It also takes up part of the buffer pool. Avoids the extra I/O to read secondary indexes in.
+    *   Adaptive hash index: Supplements InnoDB’s B-Tree indexes with fast hash lookup tables like a cache. Slight performance penalty for misses, also adds maintenance overhead of updating it. Hash collisions cause AHI rebuilding for large DBs.
+    *   Log buffer: Holds log data before flush to disk.
+
+        Size of each above memory is configurable, and impacts performance a lot. Requires careful analysis of workload, available resources, benchmarking and tuning for optimal performance.
+
+*   Disk:
+    *   Tables: Stores data within rows and columns.
+    *   Indexes: Helps find rows with specific column values quickly, avoids full table scans.
+    *   Redo Logs: all transactions are written to them, and after a crash, the recovery process corrects data written by incomplete transactions and replays any pending ones.
+    *   Undo Logs: Records associated with a single transaction that contains information about how to undo the latest change by a transaction.
+