Dynamic SQL with EXECUTE in PostgreSQL - Time & Space Complexity

Time Complexity: Dynamic SQL with EXECUTE

O(n)

Understanding Time Complexity

When using dynamic SQL with EXECUTE in PostgreSQL, it's important to understand how the time to run the query changes as the input grows.

We want to know how the number of operations grows when the dynamic query runs on larger data.

Scenario Under Consideration

Analyze the time complexity of the following dynamic SQL snippet.

DECLARE
  sql_text TEXT;
  result RECORD;
BEGIN
  sql_text := 'SELECT * FROM users WHERE age > ' || min_age;
  FOR result IN EXECUTE sql_text LOOP
    -- process each row
  END LOOP;
END;
    

This code builds a query string based on a variable and then runs it to fetch rows from the users table where age is greater than a given minimum.

Identify Repeating Operations

Look for repeated actions that affect time.

• Primary operation: Looping over each row returned by the dynamic query.
• How many times: Once for each row matching the condition in the users table.

How Execution Grows With Input

The number of rows returned depends on how many users have age greater than the given minimum.

Input Size (n)	Approx. Operations
10	About 10 rows processed
100	About 100 rows processed
1000	About 1000 rows processed

Pattern observation: As the number of matching rows grows, the loop runs more times, increasing work roughly in direct proportion.

Final Time Complexity

Time Complexity: O(n)

This means the time grows linearly with the number of rows returned by the dynamic query.

Common Mistake

[X] Wrong: "Dynamic SQL always runs in constant time because it just builds a string and executes it once."

[OK] Correct: The time depends on how many rows the query returns and processes, not just on building the string.

Interview Connect

Understanding how dynamic SQL execution time grows helps you explain performance considerations clearly and shows you can reason about real database workloads.

Self-Check

"What if the dynamic query included a JOIN with another large table? How would that affect the time complexity?"

Practice

1. What is the main purpose of using EXECUTE in PostgreSQL dynamic SQL?

easy

A. To run SQL commands that are created during the execution of a program

B. To permanently store SQL commands in the database

C. To create new tables automatically

D. To optimize static SQL queries for faster execution

Solution

1. Step 1: Understand dynamic SQL concept

   Dynamic SQL means building SQL commands as text during program run time, not fixed in advance.

2. Step 2: Role of EXECUTE

   EXECUTE runs these dynamically created SQL commands inside PostgreSQL.

3. Final Answer:

   To run SQL commands that are created during the execution of a program -> Option A

4. Quick Check:

   Dynamic SQL = EXECUTE runs built queries [OK]

Hint: EXECUTE runs SQL built as text during program run [OK]

Common Mistakes:

• Thinking EXECUTE stores queries permanently
• Confusing EXECUTE with static SQL execution
• Assuming EXECUTE creates tables automatically

2. Which of the following is the correct syntax to execute a dynamic SQL query stored in a variable sql_query inside a PL/pgSQL function?

easy

A. EXECUTE sql_query;

B. RUN sql_query;

C. EXEC sql_query;

D. PERFORM sql_query;

Solution

1. Step 1: Recall EXECUTE syntax in PL/pgSQL

   In PL/pgSQL, the correct command to run a dynamic SQL string is EXECUTE followed by the variable.

2. Step 2: Check other options

   RUN and EXEC are not valid PostgreSQL commands; PERFORM runs a static query, not dynamic SQL string.

3. Final Answer:

   EXECUTE sql_query; -> Option A

4. Quick Check:

   EXECUTE runs dynamic SQL string [OK]

Hint: Use EXECUTE to run dynamic SQL strings in PL/pgSQL [OK]

Common Mistakes:

• Using RUN or EXEC which are invalid in PostgreSQL
• Confusing PERFORM with EXECUTE for dynamic SQL
• Missing semicolon after EXECUTE statement

3. Consider this PL/pgSQL snippet:

DECLARE
  table_name text := 'users';
  rec_count int;
BEGIN
  EXECUTE 'SELECT count(*) FROM ' || table_name INTO rec_count;
  RETURN rec_count;
END;

What will this function return when called if the users table has 10 rows?

medium

A. NULL

B. An error because of missing quotes

C. 0

D. 10

Solution

1. Step 1: Understand dynamic SQL concatenation

   The query string becomes 'SELECT count(*) FROM users', which is valid SQL.

2. Step 2: EXECUTE runs the query and stores result

   EXECUTE runs the query and puts the count into rec_count variable.

3. Final Answer:

   10 -> Option D

4. Quick Check:

   Count rows in users = 10 [OK]

Hint: Concatenate table name safely, EXECUTE runs query, INTO stores result [OK]

Common Mistakes:

• Expecting error due to missing quotes around table name
• Assuming rec_count stays NULL without assignment
• Confusing dynamic SQL with static SQL syntax

4. What is the error in this PL/pgSQL code snippet?

DECLARE
  col_name text := 'age';
  val int := 30;
BEGIN
  EXECUTE 'SELECT * FROM people WHERE ' || col_name || ' = val';
END;

medium

A. Incorrect variable declaration syntax

B. Missing semicolon after EXECUTE statement

C. The variable val is not substituted inside the dynamic SQL string

D. EXECUTE cannot be used inside BEGIN...END blocks

Solution

1. Step 1: Analyze dynamic SQL string construction

   The string becomes 'SELECT * FROM people WHERE age = val', where val is literal text, not variable value.

2. Step 2: Understand variable substitution in EXECUTE

   Variables must be concatenated or passed using USING clause to substitute values properly.

3. Final Answer:

   The variable val is not substituted inside the dynamic SQL string -> Option C

4. Quick Check:

   Variables need USING or concatenation in EXECUTE [OK]

Hint: Use USING or concatenate variables in EXECUTE strings [OK]

Common Mistakes:

• Assuming variables auto-substitute inside strings
• Forgetting to use USING clause with EXECUTE
• Thinking EXECUTE can't be inside BEGIN...END

5. You want to write a PL/pgSQL function that takes a table name and a column name as inputs and returns the maximum value in that column. Which approach correctly uses dynamic SQL with EXECUTE to achieve this safely?

hard

A. EXECUTE 'SELECT max(' || col_name || ') FROM ' || table_name INTO result;

B. EXECUTE format('SELECT max(%I) FROM %I', col_name, table_name) INTO result;

C. EXECUTE 'SELECT max($1) FROM $2' USING col_name, table_name INTO result;

D. EXECUTE 'SELECT max(:col_name) FROM :table_name' INTO result;

Solution

1. Step 1: Understand safe dynamic SQL construction

   Using format() with %I safely quotes identifiers like column and table names to avoid SQL injection.

2. Step 2: Check other options for safety and correctness

   EXECUTE 'SELECT max(' || col_name || ') FROM ' || table_name INTO result; concatenates strings directly (unsafe). EXECUTE 'SELECT max($1) FROM $2' USING col_name, table_name INTO result; uses placeholders incorrectly for identifiers. EXECUTE 'SELECT max(:col_name) FROM :table_name' INTO result; uses invalid syntax.

3. Final Answer:

   EXECUTE format('SELECT max(%I) FROM %I', col_name, table_name) INTO result; -> Option B

4. Quick Check:

   Use format() with %I for identifiers in EXECUTE [OK]

Hint: Use format() with %I to safely insert identifiers in EXECUTE [OK]

Common Mistakes:

• Concatenating identifiers without quoting
• Using placeholders for table/column names
• Incorrect EXECUTE syntax with named parameters