Design innovative, scalable, and highly available cloud database solutions →hardMultiple ChoiceObjective-mapped
PCDE Practice Question: Design innovative, scalable, and highly available cloud database solutions
A company is using Cloud Spanner and needs to add a new column to an existing table that has billions of rows. The column must have a default value of 0. The team is concerned about downtime. Which approach should they take to add the column with zero downtime?
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 DEFAULT 0; the operation is online and non-blocking
Cloud Spanner supports online schema changes that are non-blocking. Adding a column with a DEFAULT value is an online operation that does not block reads or writes, and does not require copying data. The new column is added with the default value for existing rows at read time, not by backfilling.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Take the application offline, run ALTER TABLE to add the column, then bring the application back online
Why it's wrong here
Unnecessary downtime; Spanner supports online schema changes.
- ✗
Create a new table with the column, copy data using Dataflow, then rename the tables
Why it's wrong here
This is overly complex and involves downtime during the rename; Spanner supports direct online schema changes.
- ✗
Add the column without a default, then run a batch update to set the default value
Why it's wrong here
This approach requires a separate batch operation that may cause performance impact, while Spanner allows online addition with default.
- ✓
Use ALTER TABLE ADD COLUMN with DEFAULT 0; the operation is online and non-blocking
Why this is correct
Spanner schema changes are non-blocking; adding a column with a default is immediate and does not cause downtime.
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