- A
Create a new table with the column and copy data over
Why wrong: This approach requires downtime or complex dual-write logic.
- B
Execute ALTER TABLE Products ADD COLUMN discount FLOAT64
Spanner allows DDL changes while the table remains fully available.
- C
Create a secondary index that includes the new column
Why wrong: Indexes do not add a column to the base table.
- D
Define a generated column based on an existing column
Why wrong: Generated columns are computed and cannot be directly updated.
Question 124 of 1,000
PCDE Design and implement database schemas Practice Question
This PCDE practice question tests your understanding of design and implement database schemas. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 Cloud Spanner database needs to add a column 'discount' to the 'Products' table without any downtime. The table is actively used. What is the correct 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
✓
Execute ALTER TABLE Products ADD COLUMN discount FLOAT64
Option B is correct because Cloud Spanner supports online schema changes via ALTER TABLE without downtime. The operation is performed asynchronously in the background, allowing the table to remain fully available for reads and writes. The new column 'discount' is automatically populated with NULL for existing rows. Option A is incorrect because creating a new table and copying data introduces downtime and complexity. Option C is incorrect because a secondary index cannot add a column; it only indexes existing columns. Option D is incorrect because a generated column is derived from other columns, not used to add a new independent column.
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.
- ✗
Create a new table with the column and copy data over
Why it's wrong here
This approach requires downtime or complex dual-write logic.
- ✓
Execute ALTER TABLE Products ADD COLUMN discount FLOAT64
Why this is correct
Spanner allows DDL changes while the table remains fully available.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Create a secondary index that includes the new column
Why it's wrong here
Indexes do not add a column to the base table.
- ✗
Define a generated column based on an existing column
Why it's wrong here
Generated columns are computed and cannot be directly updated.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may assume schema changes require downtime or data migration in a distributed database, but Cloud Spanner's online schema change capability allows ALTER TABLE to be executed without blocking reads or writes.
Detailed technical explanation
How to think about this question
Cloud Spanner uses a distributed, synchronous replication architecture where schema changes are applied as a multi-phase operation: first, the new column metadata is propagated to all nodes, then the column is made available for writes, and finally, backfill of existing rows occurs lazily. This ensures no locks are held on the table during the ALTER TABLE statement, allowing concurrent transactions to proceed without interruption.
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.
Visual reference
What to study next
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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: Execute ALTER TABLE Products ADD COLUMN discount FLOAT64 — Option B is correct because Cloud Spanner supports online schema changes via ALTER TABLE without downtime. The operation is performed asynchronously in the background, allowing the table to remain fully available for reads and writes. The new column 'discount' is automatically populated with NULL for existing rows. Option A is incorrect because creating a new table and copying data introduces downtime and complexity. Option C is incorrect because a secondary index cannot add a column; it only indexes existing columns. Option D is incorrect because a generated column is derived from other columns, not used to add a new independent column.
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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