Description

When SQL queries contain large IN clauses with thousands of parameters (e.g., UUIDs), the SQL panel either:

• Freezes for 10-18+ seconds (sqlparse < 0.5.5)
• Crashes with SQLParseError (sqlparse >= 0.5.5)

This is a common pattern when fetching IDs from one query and filtering by them in another:

job_ids = list(Campaign.objects.filter(...).values_list('job_id', flat=True))
# job_ids contains 5000+ UUIDs

applications = JobApplication.objects.filter(job_id__in=job_ids)
# Database query is fast, but debug toolbar freezes/crashes

Behavior Matrix

Benchmark Results (sqlparse 0.5.3)

Root Cause

The bottleneck is in debug_toolbar/panels/sql/utils.py:

@lru_cache(maxsize=128)
def parse_sql(sql, *, simplify=False):
    stack = get_filter_stack(simplify=simplify)
    return "".join(stack.run(sql))  # sqlparse tokenizes entire SQL string

For a query with 5000 UUIDs:

• SQL string length: ~170,000 characters
• Token count: ~15,000+ tokens
• Each token processed by filters (indent, bold keywords, HTML escape)

The LRU cache doesn't help because each query with different parameters is a cache miss.

Steps to Reproduce

A minimal reproduction project is available:
https://github.com/kkm-horikawa/debug-toolbar-perf-issue

git clone https://github.com/kkm-horikawa/debug-toolbar-perf-issue.git
cd debug-toolbar-perf-issue
uv sync
uv run python manage.py migrate
uv run python manage.py runserver
# Visit http://127.0.0.1:8000/ and click on test links

Current Workaround

DEBUG_TOOLBAR_CONFIG = {
    'PRETTIFY_SQL': False,  # Disables ALL SQL formatting
}

This is suboptimal because it disables formatting for all queries, not just problematic ones.

Proposed Solution

I have submitted PR #2291 that:

1. Adds SQL_PRETTIFY_MAX_LENGTH setting (default 50000) to skip formatting for long queries
2. Catches SQLParseError from sqlparse >= 0.5.5 gracefully
3. Shows informative message with SQL preview when formatting is skipped

This provides:

• Normal formatting for typical queries
• Graceful degradation for extreme cases
• Configurable threshold

Related Issues

• Some SQL queries make debug toolbar rendering very slow #1402 (closed as stale - this issue provides the requested reproduction steps)

Environment

• Python 3.11+
• Django 5.0+
• django-debug-toolbar 5.2.0 / 6.1.0
• sqlparse 0.5.3 / 0.5.5+