I "Big Tech" -miljöer (du vet, den typ med massor av användare, massiva datamängder och snabbt utvecklande krav), förlita sig på databaserUNIQUE INDEX constraints to prevent duplicate data—unless it's for something like financial reconciliation where every penny must be exact—honestly, might not be as effective as you think. Plus, the cost of maintaining them can be surprisingly high. A better approach is often to handle the bulk of deduplication logic at the application layer. If you can avoid using a database unique index, consider doing so, or at least think it through very carefully before implementing one.

Varför började jag tänka om unika index? för att jag blev bränd.

Databas unika index låter ganska tillförlitligt, eller hur? den sista försvarslinjen mot dataduplikation. Jag brukade tänka så också. När ett fält i en tabell behövde vara unikt, skulle jag slumpmässigt slå ett unikt index på det.

Tills verkligheten gav mig ett hårt väckarklocka.

För länge sedan, när mitt hår var mycket fylligare, var jag tvungen att lägga till ett komposit unikt index till ett bord med tiotals miljoner rader (säg, för fält somtenant_idochis_deletedbehöver vara unika tillsammans).Låter enkelt, är det inte?daysUnder denna tid var master-slave-replikationsförseningen på en bergsklättring, och vi var ständigt oroliga för potentiella tjänstehiccups.

Sedan fanns det en annan besvärlig situation. affärsmässigt vet vi allauser@example.comochUSER@EXAMPLE.COMär effektivt samma e-post. Din ansökningskod skulle säkert normalisera dem (t.ex. till lowercase) innan du kontrollerar för dubbletter under registreringen. Men databasens unika index (som ofta är fallkänslig som standard) ser det inte på det sättet. Ibland, på grund av historiska data eller sidokanalsdata synkroniseringar som inte var ordentligt normaliserade, skulle du sluta med båda fallversioner av den "samma" e-post i databasen. I sådana fall, den unika indexet antingen "vänder ett öga" till denna företagsnivå dubbletter eller, när du försöker fixa data, dess stela regler faktiskt komma i din väg.

And don't even get me started on evolving business requirements. For instance, maybe "email uniqueness" was sufficient before, but now the requirement changes to "tenant ID + email uniqueness." Great. Application code needs to change, right? And the database's unique index has to be DROPPed och en nyCREATEd. How do you coordinate these two sets of operations? Which goes first? What if something goes wrong in between? Performing such operations on large tables feels like defusing a bomb every single time—utterly nerve-wracking.

These experiences forced me to ponder: in environments with large data volumes, high concurrency, and rapidly changing requirements, is the traditional approach to unique indexes still the right one? Have the drawbacks started to outweigh the benefits?

This article is about sharing my reflections on this.

2. UNIQUE INDEX: Why Do We Trust It So Much?

Before I dive into the complaints, let's be fair and acknowledge why unique indexes are so popular. They do have several seemingly attractive points:

1. The ultimate safeguard for data integrity: The final barrier to prevent duplicate data.
2. Lätt att implementera: Några rader av SQL när du skapar en tabell eller lägger till en DDL senare, och du är klar.
3. Schema som dokumentation: Det är markerat i schemat; det här fältet kan inte ha dubbletter.
4. A potential query performance boost: Since it's an index, queries on this key can be faster.

These benefits are indeed quite appealing for small projects, or when data volumes are manageable and business logic isn't overly complex. But things change dramatically when you enter the "battleground" of big data and rapid iteration.

3. UNIQUE INDEXUnder "Big Tech" -linsen: Är dessa fördelar fortfarande giltiga?

Let's examine each of the "benefits" mentioned above and see if they still hold up in a large-scale, fast-paced tech environment.

• "The ultimate safeguard"? Is this safeguard reliable? What exactly is it safeguarding against?

  It doesn't fully recognize business-level "duplicates"! Except the email case sensitivity issue I mentioned earlier (which could be solved by using collation but introduce more complexity in the DB layer), or phone numbers with or without +44, or usernames with or without special characters stripped... these nuances, which business logic considers "the same," are beyond the grasp of a database's simplistic "byte-for-byte identical" unique index. It can't prevent "logical duplicates" at the business layer.

  The application layer has to do the heavy lifting anyway. Since all these complex "sameness" checks must be handled in the application code (you can't just throw raw database errors at users, can you?), the application layer is the true workhorse ensuring "business data uniqueness." The database's unique index is, at best, an "auxiliary police officer" whose standards might not even align with the business rules.

  In distributed systems, it's merely a "local bodyguard." Once you shard your tables in a distributed scenario, an in-table unique index can't ensure global uniqueness. Global uniqueness then relies on ID generation services or application-level global validation. At this point, the "safeguard" provided by the local database index becomes even less significant.

  This "ultimate safeguard" might miss the mark, has limited coverage, and relying solely on it is a bit precarious.

• "Easy to implement"? One-time setup, week-long headache.

  Adding a unique index to a brand new table is indeed just one SQL statement. But more often, you're changing the rules for an old table that's been running for ages and has accumulated mountains of data. Trying to alter a unique index on a table with tens of millions of rows (e.g., changing from a single-field unique to a composite unique) could mean several minutes of table locking! Online DDL tools might save you from service downtime, but the entire process can still be lengthy, resource-intensive, and risky.

  Agile? Not so fast! In scenarios with rapid iteration, multi-region synchronization, and compliance requirements, a single unique index change at the database level can hold you up for days. So much for agility.

  So, that initial "simplicity" is like bait compared to the "hell" of modifying it later.

• "Schema as documentation"? The documentation might not match reality!

  Yes, a unique index in the table structure acts as a form of "technical documentation." But "documentation" can be misleading. If the "uniqueness" defined by this index doesn't align with the actual, more complex business rules (like the case-insensitivity example), then this "documentation" is not only useless but can also mislead future developers. If changing this "documentation" (i.e., modifying the unique index) involves an epic struggle, why not write down the business rules properly in actual design documents, wikis, or code comments? Those are far easier to update.

• "A potential query performance boost"? Is the tail wagging the dog?

  This is a common misconception, or rather, an overemphasized "added value." If you simply want to speed up queries on a specific field or set of fields, you can absolutely create a regular, non-unique index for them! A non-unique index will boost query speeds just fine, and it comes without the write overhead, DDL pains, and rigid business logic constraints of a unique index.

• Master-slave index inconsistency can instantly "paralyze" replication:

  I've seen it happen multiple times: the unique index configuration on the primary database is updated (e.g., a field is added, or a constraint is changed), but the index on the replica isn't modified in sync. Then, as soon as data changes on the primary (e.g., a row is inserted that would be considered a duplicate on the replica, or the primary can write it but the replica can't due to the incorrect/outdated index), the binlog is applied to the replica, and bam! Slave_SQL_Running: No. Replication just dies. When this happens, you get data lag, read-write splitting is affected, and it can even impact failover capabilities. What a nightmare, right?

4. Let the Application Layer Do the Job—It's What It's Good At!

Given all these issues with database unique indexes, the responsibility for ensuring data uniqueness should primarily fall on our application layer.

Fördelarna med att hantera unikhet på applikationsskiktet är många:

• Flexibel och exakt: Oavsett vad företaget definierar som en dubblett, kan vi koda logiken i enlighet därmed – fallkänslighet, formatering, komplexa förhållanden, du nämner det.
• Bättre användarupplevelse: Om en användare gör ett misstag kan vi ge tydlig och hjälpsam feedback, till exempel "Detta telefonnummer är redan registrerat.
• Efficient Early Rejection: Intercept duplicates at the service interface layer or even the gateway layer, before the data even hits the database, saving a pointless round trip.
• Interface Idempotency: This is a powerful weapon against duplicate operations. If a user double-clicks the submit button, or a network issue causes a retry, proper idempotency at the application layer ensures data isn't duplicated. A unique index can't help with this.

Conclusion

Only consider using a unique index when its benefits (usually as an absolute last-resort data backstop in extreme cases) clearly and significantly outweigh the myriad troubles it causes in complex environments with large data volumes and rapid iteration (hindering agility, operational pain). Prioritize robust application-layer uniqueness mechanisms (front-end validation, asynchronous processing, idempotency, global ID generation, etc.). As for that database unique index, avoid it if you can. If you absolutely must use one, think it through very carefully and treat it as a "specialized tool," not a "standard configuration."