$lookup join strategies: understanding the trade-offs with flexible documents

Short summary

MongoDB's $lookup joins use nested loop strategies instead of hash joins when fields are flexible (scalars, arrays, or nested objects), unlike relational databases with well-typed columns. When joining a 5M-document portfolio collection to a small FX-rates reference table, both MongoDB and DocumentDB for PostgreSQL incur similar performance costs due to type uncertainty at runtime. This deep dive explains the trade-offs between embedding data and using joins in document databases.

• •Flexible document schemas force MongoDB and DocumentDB to use nested loop joins instead of hash joins because field types are uncertain at runtime
• •Relational databases optimize joins more effectively due to scalar, well-typed columns that support diverse join algorithms
• •Understanding these architectural trade-offs informs decisions about when to embed data versus use joins for database performance

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