SELECT with PostgreSQL-specific features - Step-by-Step Execution

Concept Flow - SELECT with PostgreSQL-specific features

Start: Write SELECT query

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Parse query syntax

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Check for PostgreSQL-specific features

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Handle DISTINCT ON

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Return first row per group

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Handle RETURNING clause

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Return affected rows after INSERT/UPDATE/DELETE

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Handle FILTER clause

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Apply condition to aggregate functions

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Execute query

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Return result set

The flow shows how PostgreSQL parses a SELECT query, checks for special features like DISTINCT ON, RETURNING, and FILTER, processes them, then executes and returns results.

Execution Sample

PostgreSQL

SELECT DISTINCT ON (category) id, category, price
FROM products
ORDER BY category, price DESC;

This query selects one row per category from products, choosing the row with the highest price in each category.

Execution Table

Step	Action	Evaluation	Result
1	Parse SELECT statement	Valid syntax	Proceed to next step
2	Identify DISTINCT ON clause	DISTINCT ON (category)	Prepare to select first row per category
3	Order rows by category ASC, price DESC	Ordering applied	Rows sorted accordingly
4	Scan rows in order	First row of each category encountered	Select that row
5	Skip other rows with same category	Duplicate categories ignored	Only first per category kept
6	Return selected rows	Result set ready	Rows with unique categories and highest price returned
7	End of query execution	All rows processed	Execution complete

💡 All rows processed and first row per category returned due to DISTINCT ON

Variable Tracker

Variable	Start	After Step 3	After Step 4	After Step 5	Final
Rows	Unordered full table	Sorted by category ASC, price DESC	First row per category identified	Duplicates skipped	Result set with unique categories

Key Moments - 3 Insights

Why does DISTINCT ON select only the first row per category?

DISTINCT ON works with ORDER BY to pick the first row it encounters for each distinct category. See execution_table rows 3-5 where ordering happens first, then the first row per category is selected.

What happens if ORDER BY does not include the DISTINCT ON columns?

PostgreSQL requires ORDER BY to start with the DISTINCT ON columns to define which row is first per group. Without this, the query will error or behave unpredictably. This is implied in step 3 of the execution_table.

Is DISTINCT ON standard SQL?

No, DISTINCT ON is a PostgreSQL-specific feature. Other databases do not support it, so this query is PostgreSQL-specific as shown in the concept_flow.

Visual Quiz - 3 Questions

Look at the execution_table, at which step are duplicate categories skipped?

A Step 2

B Step 4

C Step 5

D Step 6

💡 Hint

Check the 'Action' column in execution_table row 5 about skipping duplicates

According to variable_tracker, what is the state of rows after Step 3?

A Unordered full table

B Sorted by category ASC, price DESC

C First row per category identified

D Duplicates skipped

💡 Hint

Look at the 'After Step 3' column for 'Rows' in variable_tracker

If ORDER BY did not include category, what would happen?

A PostgreSQL would raise an error

B DISTINCT ON would select random rows per category

C Query would run normally

D All rows would be returned

💡 Hint

Refer to key_moments about ORDER BY requirements for DISTINCT ON

Concept Snapshot

SELECT with PostgreSQL-specific features:
- DISTINCT ON (expression) picks first row per group
- ORDER BY must start with DISTINCT ON columns
- RETURNING returns affected rows after data changes
- FILTER applies conditions inside aggregates
Use these to write powerful PostgreSQL queries.

Full Transcript

This visual execution trace shows how PostgreSQL processes a SELECT query with the DISTINCT ON feature. The query orders rows by category and price, then selects the first row per category, effectively returning the highest priced item in each category. The flow starts with parsing, then identifying PostgreSQL-specific clauses, applying ordering, scanning rows, skipping duplicates, and returning the final result set. Variables like the row order and selection state change step-by-step. Key moments clarify why ORDER BY must include DISTINCT ON columns and that DISTINCT ON is PostgreSQL-specific. The quiz tests understanding of when duplicates are skipped, the state of rows after sorting, and the importance of ORDER BY for DISTINCT ON.