EXPLAIN output reading in PostgreSQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to show the query plan for selecting all rows from the table named employees.

PostgreSQL

EXPLAIN [1] * FROM employees;

Drag options to blanks, or click blank then click option'

ASELECT

BINSERT

CUPDATE

DDELETE

Attempts:

3 left

💡 Hint

Common Mistakes

Using INSERT, UPDATE, or DELETE after EXPLAIN instead of SELECT.

Forgetting to write SELECT after EXPLAIN.

The EXPLAIN command shows the execution plan of a query. To see the plan for selecting rows, use EXPLAIN SELECT.

2fill in blank

medium

Complete the code to get a detailed query plan with actual run times for the query selecting all rows from employees.

PostgreSQL

EXPLAIN ([1]) SELECT * FROM employees;

Drag options to blanks, or click blank then click option'

AANALYZE

BVERBOSE

CBUFFERS

DCOSTS

Attempts:

3 left

💡 Hint

Common Mistakes

Using VERBOSE or COSTS instead of ANALYZE to get actual run times.

Not including any option and missing timing details.

Adding ANALYZE runs the query and shows actual run times in the plan.

3fill in blank

hard

Fix the error in the EXPLAIN command to show the query plan with buffers information for a SELECT query.

PostgreSQL

EXPLAIN (ANALYZE, [1]) SELECT * FROM employees;

Drag options to blanks, or click blank then click option'

ATIMING

BANALYZE

CBUFFERS

DVERBOSE

Attempts:

3 left

💡 Hint

Common Mistakes

Repeating ANALYZE option twice.

Using TIMING which is deprecated in newer PostgreSQL versions.

Using VERBOSE which shows more details but not buffer info.

The BUFFERS option shows buffer usage in the query plan. It must be used with ANALYZE to see actual buffer reads and hits.

4fill in blank

hard

Fill both blanks to create a query that shows the query plan with costs and without actual execution for a SELECT from employees.

PostgreSQL

EXPLAIN ([1], [2]) SELECT * FROM employees;

Drag options to blanks, or click blank then click option'

ACOSTS

BANALYZE

CBUFFERS

DVERBOSE

Attempts:

3 left

💡 Hint

Common Mistakes

Including ANALYZE which runs the query and shows actual times.

Using BUFFERS without ANALYZE which does not show buffer info.

COSTS shows estimated costs in the plan. VERBOSE adds extra details. Without ANALYZE, the query is not run, so no actual times appear.

5fill in blank

hard

Fill all three blanks to create a query that shows the query plan with actual execution, buffers info, and verbose output for a SELECT from employees.

PostgreSQL

EXPLAIN ([1], [2], [3]) SELECT * FROM employees;

Drag options to blanks, or click blank then click option'

AANALYZE

BBUFFERS

CVERBOSE

DCOSTS

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting ANALYZE which means no actual run times.

Using COSTS instead of VERBOSE for detailed output.

Not including BUFFERS to see memory usage.

ANALYZE runs the query and shows actual times. BUFFERS shows buffer usage. VERBOSE adds detailed info about the plan nodes.

Practice

1. What does the EXPLAIN command in PostgreSQL primarily show?

easy

A. How PostgreSQL plans to execute a query

B. The exact data returned by the query

C. The syntax errors in the query

D. The database schema structure

Solution

1. Step 1: Understand the purpose of EXPLAIN

   EXPLAIN shows the query plan, which is how PostgreSQL intends to run the query.

2. Step 2: Differentiate from other commands

   It does not show actual data or errors, only the plan.

3. Final Answer:

   How PostgreSQL plans to execute a query -> Option A

4. Quick Check:

   EXPLAIN = query plan [OK]

Hint: EXPLAIN = query plan, not data or errors [OK]

Common Mistakes:

• Thinking EXPLAIN shows query results
• Confusing EXPLAIN with syntax error checks
• Assuming EXPLAIN shows database schema

2. Which of the following is the correct syntax to get the query plan for SELECT * FROM users; in PostgreSQL?

easy

A. EXPLAIN SELECT * FROM users;

B. EXPLAIN ANALYZE users SELECT *;

C. EXPLAIN FROM users SELECT *;

D. ANALYZE EXPLAIN SELECT * FROM users;

Solution

1. Step 1: Recall correct EXPLAIN syntax

   The correct syntax is EXPLAIN followed by the query.

2. Step 2: Check each option

   EXPLAIN SELECT * FROM users; matches the correct syntax. Others mix keywords incorrectly.

3. Final Answer:

   EXPLAIN SELECT * FROM users; -> Option A

4. Quick Check:

   EXPLAIN + query = correct syntax [OK]

Hint: EXPLAIN always precedes the query [OK]

Common Mistakes:

• Placing ANALYZE before EXPLAIN
• Using FROM before SELECT incorrectly
• Mixing keywords in wrong order

3. Given the EXPLAIN output below for SELECT * FROM orders WHERE customer_id = 5;, what does the line Index Scan using idx_customer_id on orders indicate?

medium

A. PostgreSQL is scanning the entire orders table

B. PostgreSQL is using an index to find matching rows

C. PostgreSQL is performing a sequential scan

D. PostgreSQL is creating a new index during query

Solution

1. Step 1: Understand 'Index Scan' meaning

   An Index Scan means PostgreSQL uses an index to quickly find rows matching the condition.

2. Step 2: Compare with other scan types

   Sequential scan means scanning all rows, which is not the case here.

3. Final Answer:

   PostgreSQL is using an index to find matching rows -> Option B

4. Quick Check:

   Index Scan = use index [OK]

Hint: 'Index Scan' means index used, not full table scan [OK]

Common Mistakes:

• Confusing Index Scan with Sequential Scan
• Thinking index is created during query
• Assuming full table scan always happens

4. You run EXPLAIN ANALYZE SELECT * FROM products WHERE price > 100; but get an error saying "relation 'products' does not exist." What is the likely cause?

medium

A. EXPLAIN ANALYZE cannot be used with WHERE clauses

B. The query syntax is incorrect for EXPLAIN ANALYZE

C. The table 'products' does not exist in the current database

D. You forgot to commit the transaction

Solution

1. Step 1: Analyze the error message

   The error says the table 'products' does not exist, meaning PostgreSQL cannot find it.

2. Step 2: Check other options

   EXPLAIN ANALYZE works with WHERE clauses and the syntax is correct. Committing transaction is unrelated.

3. Final Answer:

   The table 'products' does not exist in the current database -> Option C

4. Quick Check:

   Relation not found = missing table [OK]

Hint: Check table existence if 'relation does not exist' error appears [OK]

Common Mistakes:

• Assuming EXPLAIN ANALYZE disallows WHERE
• Blaming syntax when table is missing
• Thinking commit affects table visibility

5. You want to optimize a slow query. The EXPLAIN ANALYZE output shows a Seq Scan on a large table with a filter on a column. What is the best next step to improve performance?

hard

A. Drop the table and recreate it

B. Rewrite the query without the filter

C. Increase the work_mem setting

D. Create an index on the filtered column

Solution

1. Step 1: Understand Seq Scan impact

   A Seq Scan reads all rows, which is slow on large tables when filtering.

2. Step 2: Use index to speed filtering

   Creating an index on the filtered column lets PostgreSQL quickly find matching rows, avoiding full scan.

3. Final Answer:

   Create an index on the filtered column -> Option D

4. Quick Check:

   Seq Scan slow? Add index [OK]

Hint: Seq Scan slow? Add index on filter column [OK]

Common Mistakes:

• Removing filter instead of indexing
• Changing memory settings without indexing
• Dropping table unnecessarily