DBS-C01 Workload-Specific Database Design Practice Question
A company runs a large-scale e-commerce platform using Amazon RDS for MySQL with a Multi-AZ deployment. The database has a table 'orders' with 200 million rows. Recently, they added a new index on the 'order_date' column to improve reporting queries. After adding the index, they noticed increased write latency and occasional replication lag. The application writes new orders continuously. The table experiences about 10,000 writes per second. The DB instance is db.r5.4xlarge. The index creation was done using the ALTER TABLE statement with a default algorithm. What is the most likely cause of the increased write latency and replication lag?
⚠ Common exam trap
A common mix-up: candidates assume any index addition causes permanent write overhead (Option D), but the question describes a sudden latency spike immediately after the operation, which is characteristic of the blocking COPY algorithm, not ongoing maintenance.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The index was created using the default algorithm (COPY), which locks the table and blocks writes, causing replication lag.
The default algorithm for ALTER TABLE in MySQL is COPY, which creates a new table, copies all rows, and rebuilds indexes. During this process, the table is locked with a write lock, blocking DML operations and causing increased write latency. In a Multi-AZ deployment, the DDL is replicated to the standby, but the lock on the primary delays writes, which can manifest as replication lag when the standby applies the same blocking DDL.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The index creation DDL statement is not replicated to the standby instance, causing inconsistency.
Why it's wrong here
DDL statements are replicated; the issue is the lock on the primary.
- ✗
The instance size is insufficient for the write workload.
Why it's wrong here
The instance is db.r5.4xlarge, which is typically sufficient for 10K writes/sec; the issue appeared after index creation.
- ✓
The index was created using the default algorithm (COPY), which locks the table and blocks writes, causing replication lag.
Why this is correct
In MySQL 5.6 and 5.7, ALTER TABLE uses COPY algorithm by default, which locks the table for writes during the operation.
- ✗
The new index is causing excessive overhead on write operations due to index maintenance.
Why it's wrong here
Index maintenance adds some overhead but not typically enough to cause significant latency and lag; the problem is the creation process.
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