- A
Use 'ALTER TABLE ... ADD COLUMN ... ALGORITHM=INPLACE, LOCK=NONE'.
Why wrong: In MySQL, ALGORITHM=INPLACE may still require a rebuild and metadata lock for some column additions.
- B
Use a tool like pt-online-schema-change to perform the change with minimal impact.
pt-online-schema-change uses triggers and a shadow table to avoid locks.
- C
Create a new table with the column, copy data manually, then swap tables.
Why wrong: Manual data copying is error-prone and risky; pt-online-schema-change automates it safely.
- D
Use 'ALTER TABLE ... ADD COLUMN' directly; Cloud SQL handles it online.
Why wrong: Direct ALTER TABLE on large tables causes locks and potential downtime.
Question 565 of 1,000
PCDOE Design and Plan Database Solutions Practice Question
This PCDOE practice question tests your understanding of design and plan database solutions. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A company runs a MySQL database on Cloud SQL for an e-commerce platform. They need to add a new column to a table with millions of rows without causing downtime. What is the recommended approach?
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
✓
Use a tool like pt-online-schema-change to perform the change with minimal impact.
Option B is correct because pt-online-schema-change (or gh-ost) creates a shadow table with the new schema, incrementally copies rows using triggers or binary log replay, and then atomically swaps the tables. This avoids holding any locks on the original table, preventing downtime for an e-commerce platform with millions of rows. Cloud SQL's InnoDB does not support true online DDL for all ALTER TABLE operations, especially on large tables, making a dedicated online schema change tool the safest approach.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Use 'ALTER TABLE ... ADD COLUMN ... ALGORITHM=INPLACE, LOCK=NONE'.
Why it's wrong here
In MySQL, ALGORITHM=INPLACE may still require a rebuild and metadata lock for some column additions.
- ✓
Use a tool like pt-online-schema-change to perform the change with minimal impact.
Why this is correct
pt-online-schema-change uses triggers and a shadow table to avoid locks.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Create a new table with the column, copy data manually, then swap tables.
Why it's wrong here
Manual data copying is error-prone and risky; pt-online-schema-change automates it safely.
- ✗
Use 'ALTER TABLE ... ADD COLUMN' directly; Cloud SQL handles it online.
Why it's wrong here
Direct ALTER TABLE on large tables causes locks and potential downtime.
Common exam traps
Common exam trap: answer the scenario, not the keyword
A common trap is assuming that Cloud SQL's managed service automatically makes all DDL operations online for large tables, but MySQL's native online DDL has limitations and does not eliminate downtime without using external tools like pt-online-schema-change or gh-ost.
Detailed technical explanation
How to think about this question
Under the hood, pt-online-schema-change uses triggers on the original table to capture changes during the copy phase, while gh-ost uses the binary log stream to avoid trigger overhead. Both tools rely on a final RENAME TABLE operation, which is atomic and fast, to swap the tables. In a real-world scenario, a direct ALTER TABLE on a 100-million-row table could take hours, blocking all writes and causing timeouts for the e-commerce frontend, whereas an online schema change tool keeps the application fully available.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A cloud solutions architect for a retail company is evaluating services for a new workload. The correct answer here reflects best practice for the specific scenario described — not a general cloud recommendation. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Cloud exam questions reward reading the constraint carefully: the same technology can be right or wrong depending on the use case.
What to study next
Got this wrong? Here's your next step.
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FAQ
Questions learners often ask
What does this PCDOE question test?
Design and Plan Database Solutions — This question tests Design and Plan Database Solutions — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Use a tool like pt-online-schema-change to perform the change with minimal impact. — Option B is correct because pt-online-schema-change (or gh-ost) creates a shadow table with the new schema, incrementally copies rows using triggers or binary log replay, and then atomically swaps the tables. This avoids holding any locks on the original table, preventing downtime for an e-commerce platform with millions of rows. Cloud SQL's InnoDB does not support true online DDL for all ALTER TABLE operations, especially on large tables, making a dedicated online schema change tool the safest approach.
What should I do if I get this PCDOE question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jul 4, 2026
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