semgrep__skills/skills/code-security/rules/performance.md

title, impact

title	impact
Performance Best Practices	LOW

Performance Best Practices

This document covers performance optimizations to write efficient code. These rules identify patterns that cause unnecessary computational overhead, extra database queries, or memory inefficiency.

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Python

Django - Access Foreign Keys Directly

Use ITEM.user_id rather than ITEM.user.id to prevent running an extra query. Accessing .user.id causes Django to fetch the entire related User object just to get the ID, when the foreign key ID is already available on the model.

INCORRECT - Extra query to fetch related object:

def get_user_id(item):
    return item.user.id

CORRECT - Use the foreign key directly:

def get_user_id(item):
    return item.user_id

---

SQLAlchemy - Use count() Instead of len(all())

Using QUERY.count() instead of len(QUERY.all()) sends less data to the client since the count is performed server-side. The len(all()) approach fetches all records into memory just to count them.

INCORRECT - Fetches all records into memory:

total = len(persons.all())

CORRECT - Count performed server-side:

total = persons.count()

---

SQLAlchemy - Batch Database Operations

Rather than adding one element at a time, use batch loading to improve performance. Each individual db.session.add() in a loop can trigger separate database operations.

INCORRECT - Adding one at a time in a loop:

for song in songs:
    db.session.add(song)

CORRECT - Batch add all at once:

db.session.add_all(songs)

---

JavaScript/TypeScript

React - Define Styled Components at Module Level

By declaring a styled component inside the render method, you dynamically create a new component on every render. This forces React to discard and re-calculate that part of the DOM subtree on each render, leading to performance bottlenecks.

INCORRECT - Styled component declared inside function:

import styled from "styled-components";

function FunctionalComponent() {
  const StyledDiv = styled.div`
    color: blue;
  `
  return <StyledDiv />
}

CORRECT - Styled component declared at module level:

import styled from "styled-components";

const StyledDiv = styled.div`
  color: blue;
`

function FunctionalComponent() {
  return <StyledDiv />
}

---

Avoid Unnecessary Operations in Loops

Check array length efficiently without traversing the entire collection.

INCORRECT - Inefficient length check:

if (items.length === 0) { /* empty */ }

CORRECT - Direct comparison when possible:

if (!items.length) { /* empty */ }

For operations that require iterating, prefer built-in methods that short-circuit:

INCORRECT - Full iteration to find one item:

const found = items.filter(x => x.id === targetId)[0];

CORRECT - Short-circuit on first match:

const found = items.find(x => x.id === targetId);