Magic IndexedDB Blazor: .Where() vs .Cursor()

Published: June 02, 2025

Magic IndexedDB Blazor: .Where() vs .Cursor()

Magic IndexedDB offers two primary query interfaces in its C# Blazor wrapper: .Where() and .Cursor(). These aren't just stylistic choices—they're intentional, opinionated design tools that directly impact how your queries are interpreted, optimized, and executed.

✅ TL;DR: Use .Where() unless you're absolutely certain you need .Cursor().

🔍 The Purpose of Each Interface

.Where() – Optimized by Default

Designed to try using IndexedDB indexes wherever possible.

The engine will analyze and partition your query to find the most efficient execution path.

There is no guarantee that all of your query will be indexed, but:

There’s a non-zero chance that some or all of it will be.

The structure ensures you don’t accidentally fall into a purely cursor-based query.

⚠️ Even if your .Where() query ends up executing as a cursor behind the scenes, the interface is built to promote indexed-first thinking.

.Cursor() – Explicit Fallback Mode

Meant for scenarios where you know indexing won’t help (e.g., Contains(), case-insensitive matching, missing fields, etc).

Disables the optimization layer entirely—no partitioning, no compound index checks.

Tells the engine: "Don’t try to optimize this—I’m manually opting into the cursor engine."

Enables syntactic flexibility that .Where() restricts to avoid misuse (e.g., .TakeLast(), deeper string functions, nullable tricks).

✅ Use .Cursor() when you're intentionally sacrificing index performance for expressive power.

🧠 Design Philosophy Behind the Interfaces

.Where() encourages best habits by forcing developers to think in terms of performance and index safety.

Its interfaces restrict syntax in ways that map cleanly to what IndexedDB can optimize.

.Cursor() acknowledges intent—you're declaring, "I know this can't be optimized, and that’s okay."

It lifts the syntactic guardrails and lets you use advanced querying power consciously.

Think of .Where() as your high-performance lane, and .Cursor() as your off-road vehicle—powerful, flexible, but slower.

🔄 Interface Differences: Summary Table

Feature	`.Where()`	`.Cursor()`
Index Optimization	✅ Attempted	❌ Disabled
Query Partitioning	✅ Enabled	❌ Disabled
Syntax Restrictions	✅ Enforced	❌ Lifted
Allows `.TakeLast()`	✅ Yes (via transform)	✅ Yes
Guarantees Indexed Execution	❌ No	❌ No
Guarantees Cursor Execution	❌ No	✅ Yes
Use Case	Safe, optimized querying	Intentional fallback, complex logic

🚀 How to Decide

Does my query use indexed fields? → Use .Where()

Am I doing anything like Contains(), EndsWith(), or case-insensitive matching? → Use .Cursor()

Am I unsure if a field is indexed but want the best chance at performance? → Use .Where()

Am I doing something syntactically unsupported in .Where() (like .TakeLast())? → Use .Cursor()

📚 Learn How the Engine Works

To fully understand how your queries are partitioned and optimized under the hood, read:

How LINQ to IndexedDB Works (Engine Fundamentals)

That documentation covers:

Query partitioning

Index fallback logic

How cursor queries are optimized (yes, even those!)

Real-world examples of how your intent is preserved while being made performant

🧠 Final Advice

Always prefer .Where() unless you have a compelling reason.

Use .Cursor() only when you fully understand the implications.

Remember: Magic IndexedDB is not IndexedDB. It’s a system built to help you query IndexedDB like a modern database—safely, expressively, and intentionally.

💬 If you’re making a Cursor query, it’s powerful—but you should always ask yourself: Is there no way I can make this indexed?

Stay fast. Stay safe. Query smart. 💡