- A
Use ALTER TABLE ADD COLUMN without a default value and then update rows in batches
Why wrong: Requires a separate update operation; can lead to temporary schema inconsistency.
- B
Use ALTER TABLE ADD COLUMN with a non-null default value
Cloud Spanner applies the default immediately without scanning or rewriting rows.
- C
Create a new table and use batch operations to copy data
Why wrong: Time-consuming and requires careful synchronization.
- D
Drop and recreate the table with the new column
Why wrong: Causes complete downtime and data loss if not carefully managed.
Question 909 of 1,000
Cloud Spanner Online Schema Changes — Add Non-Nullable Column with Default
This PCDE practice question tests your understanding of design and implement database schemas. 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.
Your team needs to add a new non-nullable column with a default value to a large Cloud Spanner table. The table has thousands of simultaneous writes per second. Which approach minimizes downtime and resource usage?
Quick Answer
The answer is to use ALTER TABLE ADD COLUMN with a non-null default value, because Cloud Spanner performs this as an online schema change that applies the default metadata without rewriting existing rows, making it the only approach that avoids downtime and heavy resource usage even under thousands of simultaneous writes per second. This tests your understanding of Spanner’s online DDL capabilities, a key topic on the Google Professional Cloud Database Engineer exam where the common trap is assuming you must add a nullable column first and then backfill, which wastes resources and risks inconsistency. The technical concept here is that Spanner stores the default value at the schema level, so the column is immediately non-nullable for new rows and logically present for existing rows without a costly full-table scan. Remember the memory tip: “Default at DDL, no rows to fill” — if you can set a non-null default in the ALTER statement, Spanner handles it online without rewriting a single row.
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 ALTER TABLE ADD COLUMN with a non-null default value
Option B is correct because adding a non-nullable column with a default value in Cloud Spanner is a metadata-only operation that does not rewrite existing rows or block reads/writes. This minimizes downtime and resource usage even under thousands of concurrent writes per second, as the default value is applied logically at read time.
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 without a default value and then update rows in batches
Why it's wrong here
Requires a separate update operation; can lead to temporary schema inconsistency.
- ✓
Use ALTER TABLE ADD COLUMN with a non-null default value
Why this is correct
Cloud Spanner applies the default immediately without scanning or rewriting rows.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Create a new table and use batch operations to copy data
Why it's wrong here
Time-consuming and requires careful synchronization.
- ✗
Drop and recreate the table with the new column
Why it's wrong here
Causes complete downtime and data loss if not carefully managed.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume adding a column with a default value requires a full table scan or row updates, similar to traditional databases, but Cloud Spanner handles this as a schema-only change without data movement.
Detailed technical explanation
How to think about this question
Under the hood, Cloud Spanner stores default values for new non-nullable columns as part of the schema version, not in each row; existing rows return the default value on read without physical storage. This behavior leverages Spanner's TrueTime and distributed commit protocol to ensure consistency without locking, making it ideal for high-throughput OLTP workloads. A real-world scenario is adding an 'is_active' column defaulting to TRUE to a user table with millions of rows—this completes in seconds with zero impact on ongoing transactions.
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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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FAQ
Questions learners often ask
What does this PCDE question test?
Design and implement database schemas — This question tests Design and implement database schemas — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Use ALTER TABLE ADD COLUMN with a non-null default value — Option B is correct because adding a non-nullable column with a default value in Cloud Spanner is a metadata-only operation that does not rewrite existing rows or block reads/writes. This minimizes downtime and resource usage even under thousands of concurrent writes per second, as the default value is applied logically at read time.
What should I do if I get this PCDE 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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