EXPLAIN output reading in PostgreSQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the EXPLAIN command do in PostgreSQL?

It shows the query plan that PostgreSQL will use to run a SQL query. This helps understand how the database processes the query.

beginner

In EXPLAIN output, what does Seq Scan mean?

It means PostgreSQL reads the whole table row by row to find matching data. This is simple but can be slow for big tables.

intermediate

What is the significance of cost= in EXPLAIN output?

The cost shows an estimate of how expensive the query step is. Lower cost means faster or cheaper to run.

beginner

What does Index Scan indicate in EXPLAIN output?

It means PostgreSQL uses an index to quickly find rows instead of scanning the whole table. This is usually faster.

intermediate

Why is understanding Rows in EXPLAIN output important?

It shows how many rows PostgreSQL expects to process at each step. This helps identify if the query is efficient or not.

What does the EXPLAIN command show in PostgreSQL?

A The server status

B The query result data

C The database schema

D The query execution plan

EXPLAIN shows the query execution plan, not the actual data or schema.

In EXPLAIN output, what does Seq Scan mean?

A Reading the whole table row by row

B Joining two tables

C Sorting the data

D Using an index to find rows

Seq Scan means scanning the entire table sequentially.

What does a lower cost= value in EXPLAIN output suggest?

A The query step is faster or cheaper

B The query will return more rows

C The query step is more expensive

D The query will fail

Lower cost means the database estimates the step to be faster or less resource-heavy.

What does Index Scan in EXPLAIN output indicate?

A The table is scanned fully

B An index is used to find rows quickly

C The query is sorted

D The query is cached

Index Scan means PostgreSQL uses an index to speed up row lookup.

Why is the Rows value in EXPLAIN output useful?

A It shows the number of tables

B It shows the number of columns

C It shows how many rows are expected to be processed

D It shows the query execution time

Rows indicate how many rows the database expects to handle at each step.

Explain what information the EXPLAIN command provides and why it is useful.

Think about how EXPLAIN helps you understand what the database does behind the scenes.

Describe the difference between Seq Scan and Index Scan in EXPLAIN output.

Consider how the database finds data in each case.