PCDE Monitor and optimize database performance — 10 Questions

Question 1

A user runs the above command and expects a row to be returned because the user exists. Which index is missing?

Accepted Answer

No index needed, query scans full table.. If the user exists, a full table scan will find the row. An index on Email would improve performance but is not required for correctness. The query will return the row regardless of indexes, as long as the row exists in the table. Options A and B incorrectly suggest that a missing index prevents the row from being returned, which is not true. Option C is unnecessary because a composite index is not needed for this simple equality filter.

Answer

Primary key index on Users(Email)

Answer

Index on Users(Email)

Answer

Composite index on Users(Email, UserId)

Question 2

An alert is configured to trigger when the number of queries exceeding the 'log_min_duration_statement' threshold suddenly spikes. What is this alert monitoring?

Accepted Answer

High number of slow queries. The alert monitors a high number of slow queries, as the threshold 'log_min_duration_statement' in PostgreSQL determines which queries are logged as slow. A spike in these slow query logs directly indicates performance degradation from inefficient SQL, missing indexes, or lock contention, making it a key metric for database performance monitoring.

Answer

High disk I/O

Answer

High number of database connections

Answer

High CPU usage

Question 3

You are managing a Cloud SQL for PostgreSQL instance that is experiencing high CPU usage and slow query performance. You notice that the database has a high number of idle-in-transaction connections. Which immediate action should you take to reduce CPU load without disrupting active transactions?

Accepted Answer

Set the cloudsql.enable_idle_in_transaction_session_timeout flag to true and configure idle_in_transaction_session_timeout.. Setting the `cloudsql.enable_idle_in_transaction_session_timeout` flag to true and configuring `idle_in_transaction_session_timeout` allows Cloud SQL to automatically terminate idle-in-transaction connections after a specified timeout, reducing CPU load without manually killing connections or disrupting active transactions. This is a built-in, non-disruptive mechanism that targets only connections that are holding resources while idle, freeing up CPU and memory for active queries.

Answer

Use VPC firewall rules to block new connections until the issue resolves.

Answer

Kill all idle-in-transaction connections using pg_terminate_backend.

Answer

Set a statement_timeout at the session level for new connections.

Question 4

A team is deploying a new application on Google Kubernetes Engine (GKE) that uses Cloud Spanner. They want to minimize latency for read operations. Which Spanner configuration should they use?

Accepted Answer

Use a single-region instance and configure the leader preference to the application's zone.. Using a single-region Cloud Spanner instance with leader preference set to the zone where the application runs ensures that the leader replica is in the same zone as the application. Strongly consistent reads are served from the leader, so this configuration minimizes read latency by avoiding cross-zone network round trips. Option B is incorrect because a regional instance does not have separate 'read replicas'; all replicas in a regional instance are read-write, and the leader is already in the same region, but specifying 'read replicas' is misleading and not a valid Spanner configuration. Multi-region options (A and D) introduce cross-region latency for strongly consistent reads.

Answer

Use a multi-region configuration with default leader preference set to the region where the application runs.

Answer

Use a regional instance with read replicas in the same region.

Answer

Use a single-region instance and enable read-only replicas in multiple zones.

Question 5

Your Cloud SQL for MySQL instance is experiencing intermittent performance degradation. You suspect that the issue is due to a sudden spike in connections from a specific application. Which metric and monitoring approach would best help you correlate the connection spike with performance degradation?

Accepted Answer

Monitor 'cloudsql.googleapis.com/database/mysql/threads/threads_connected' and correlate with CPU utilization and query latency.. The 'threads_connected' metric directly measures the number of active connections to the MySQL instance. Correlating this with CPU utilization and query latency allows you to pinpoint whether a sudden spike in connections is causing resource contention and degraded query performance, which is the exact scenario described.

Answer

Monitor 'cloudsql.googleapis.com/network/received_bytes_count' and compare with connection count.

Answer

Monitor 'cloudsql.googleapis.com/database/mysql/replication/seconds_behind_master' and compare with query latency.

Answer

Monitor 'cloudsql.googleapis.com/instance/uptime' and check for instance restarts during degradation.

Question 6

You are running a production workload on Cloud Bigtable and notice that read latency has increased. Upon reviewing the monitoring dashboard, you see that CPU utilization is below 50% but the number of active tablets is high. What is the most likely cause of the increased read latency?

Accepted Answer

There are too many tablets, causing increased metadata operations and slower reads.. In Cloud Bigtable, each tablet is a contiguous range of rows managed by a tablet server. When the number of active tablets is high, the tablet server must perform more metadata operations (e.g., splitting, merging, and serving multiple tablets) which increases per-request overhead and can degrade read latency. This is true even when CPU utilization is below 50%, because the overhead is not purely CPU-bound but involves increased I/O and coordination.

Answer

Read requests are being throttled due to exceeding IOPS limits.

Answer

A hot node is throttling read requests.

Answer

The cluster is underprovisioned, causing resource contention.

Question 7

A developer has deployed a new version of an application that uses Cloud SQL. After the deployment, you notice a sharp increase in the number of slow queries. What should you do first to identify the problematic queries?

Accepted Answer

Use Cloud SQL Query Insights to identify the queries with the highest latency and examine their execution plans.. Cloud SQL Query Insights is the recommended first step for diagnosing slow queries because it provides built-in query monitoring, latency breakdowns, and execution plans without additional configuration. It directly surfaces the queries with the highest latency, allowing you to examine their execution plans to identify root causes such as missing indexes or inefficient joins.

Answer

Check the slow query log in Cloud Logging and look for queries with high rows_examined.

Answer

Increase the instance tier to reduce the impact of slow queries.

Answer

Enable the general query log and parse the log file to find slow queries.

Question 8

Which TWO actions can help reduce the number of read replicas needed for a Cloud SQL for PostgreSQL instance that serves a read-heavy workload?

Accepted Answer

Implement connection pooling to reuse database connections.. Connection pooling reduces the overhead of establishing new database connections, which can consume significant CPU and memory resources on the primary instance. By reusing existing connections, the primary instance can handle more read requests without needing additional read replicas to offload the connection management load. This directly reduces the number of replicas required for a read-heavy workload.

Answer

Enable synchronous replication on all read replicas.

Answer

Use smaller machine types for read replicas.

Answer

Increase the max_connections parameter on the primary instance.

Question 9

Which THREE factors should you consider when configuring Cloud SQL for MySQL query caching to optimize performance?

Accepted Answer

High write workloads can lead to frequent cache invalidation, reducing its effectiveness.. In Cloud SQL for MySQL, high write workloads cause frequent updates to tables, which invalidates the query cache entries for those tables. This means the cache must be repopulated often, reducing the hit rate and potentially adding overhead from cache maintenance, making it less effective for performance optimization.

Answer

Query cache uses disk storage for cached results.

Answer

Prepared statements always bypass the query cache.

Question 10

Your Cloud SQL for PostgreSQL instance is experiencing intermittent slowdowns during peak hours. You notice that the CPU utilization spikes to 80% and the number of connections increases. The application team confirms they are not running any new queries. What should you do first to diagnose the issue?

Accepted Answer

Use Cloud SQL Insights to analyze query performance and wait statistics.. Cloud SQL Insights provides built-in query performance monitoring and wait statistics that can pinpoint the root cause of intermittent slowdowns without making changes. Since CPU spikes and increased connections are symptoms, not the cause, analyzing wait events (e.g., CPU, IO, lock contention) directly reveals which queries or resources are bottlenecked. This is the first diagnostic step before any scaling or configuration changes.

Answer

Increase the machine type of the Cloud SQL instance to add more CPU.

Answer

Enable connection pooling to reduce the number of connections.

Answer

Set a maximum connection limit and reduce the connection lifetime.