9 pieces
MongoDB, in more detail than the manual
Query plans that look fine and are not, schema choices that only show their price at scale, and the gap between what the documentation says and what the server does on the version you are actually running. Everything with a number in it comes from a run you can reproduce.
There is a lot of MongoDB writing on the internet and most of it restates the manual. These pieces try to be the other thing: the case the documentation stops short of, the version where the accepted advice went out of date, and the cost of a schema decision in milliseconds rather than in adjectives. Where a piece claims a number, the script that produced it is published with it, so you can disagree with the result rather than with us.
Several of them end at one of the free browser tools, because a diagnostic you can run against your own output beats a screenshot of somebody else's.
RSS · Comparisons with other MongoDB clients · Who writes this
Diagnostics
A symptom, the output that explains it, and the tool that shows you yours.
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7 September 2026 Extended JSON, and where $date and $oid quietly change your data JSON has no date type and no 64-bit integer. Extended JSON has both, in two modes, and relaxed mode loses less than people think and more than they check. -
7 September 2026 A compound index in the wrong order is a sort you did not ask for The same three fields in a different order decides whether the index serves your sort or the server does it in memory. The plan says which you have. -
7 September 2026 Your query uses an index and is still slow An IXSCAN in the plan is not the finish line. The number that matters is how many documents the server had to read to return the ones you asked for.
Errors
What the message actually means, and what to change.
Measured
Numbers from a run you can reproduce, with the script published beside them.
Reference
Version-precise answers to questions the manual leaves open.
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7 September 2026 What each MongoDB release added to the aggregation language, from 5.0 on The answer to "why does $percentile not work" is almost always the server version. Here is what arrived in each release from 5.0 to 8.3. -
7 September 2026 The 100 MB aggregation limit stopped failing your query in MongoDB 6.0 Most guides say a stage over 100 MB errors unless you pass allowDiskUse. That has been wrong since MongoDB 6.0, and the new failure is quiet.
AI access
Giving an assistant a database connection, and what it can still reach.
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13 September 2026 What a MongoDB MCP server can reach, before you point one at production The official server writes unless you pass --readOnly, its confirmations depend on the client supporting elicitation, and its Atlas tools create users. -
7 September 2026 Read-only MongoDB access for an AI agent, and what still gets through A read-only flag that works by reading the query text can be walked past in four lines. Where the check runs matters more than what it is called.