init docs

FerretDB · noisersup · Apr 12, 2023 · Mar 31, 2023 · Mar 31, 2023 · Apr 4, 2023
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+# Query pushdown
+
+Pushdown is the method of optimizing a query by reducing the amount of data read and processed. 
+It saves memory space, and network bandwidth, and reduces the query execution time by not prefetching 
+unnecessary data to the database management system.
+
+As FerretDB mission is to become true open-source MongoDB-compatible database, it's important
+to handle all operations, comparisons, data types, and commands in the same fashion as MongoDB.
+
+Doing so can be really challenging on different database backends. For example, it's difficult to
+make PostgreSQL abide BSON types comparison order. There are other things like the fact that PostgreSQL
+uses numeric type to floating-point numbers, which is not IEEE 754 compilant (like MongoDB), and creates
+some comparison issues on larger values.
+
+Even, if it's possible, the SQL query that checks for all of the logic might be huge and really hard to maintain.
+
+Because of that, FerretDB fetch all necessary data from collection and applies filters on it's own. 
+That solution allows to implement all complicated logic safely, easily and quickly. Maintainers are not overwhelmed
+by database backend specifics, which makes FerretDB a great, universal solution to implement new backends.
+
+The downside of this is obvious - fetching much data from collection on every single query can be inefficient and
+time-consuming (especially for larger collections).
+
+Query pushdown is a great compromise between complexity and performance.
+It allows to handle all MongoDB logic without many issues related to underyling database architecture,
+while keeping good query performance.
+
+
+The following table shows all operators and types that FerretDB pushdowns on PostgreSQL backend.
+If filter use specified type and operator, FerretDB will prefetch less data and the query will be more performent.
+
+As pushdown approach allows developers to implement query optimizations later, the table will 
+be updated with every change.
+
+If your application requires better performance for specific operation, feel free to join slack channel.
+
+|            | Object   | Array   | Double                 | String   | Binary   | ObjectID   | Boolean   | Date   | Null   | Regex   | Integer   | Timestamp   | Long                   |
+| ---------- | -------- | ------- | --------               | -------- | -------- | ---------- | --------- | ------ | ------ | ------- | --------- | ----------- | ------                 |
+| `=`        | ✖️        | ✖️       | ⚠️ <sub>[[1]](#1)</sub> | ✅       | ✖️        | ✅         | ✅        | ✅     | ✖️      | ✖️       | ✅        | ✖️           | ⚠️ <sub>[[1]](#1)</sub> |
+| `$eq`      | ✖️        | ✖️       | ⚠️ <sub>[[1]](#1)</sub> | ✅       | ✖️        | ✅         | ✅        | ✅     | ✖️      | ✖️       | ✅        | ✖️           | ⚠️ <sub>[[1]](#1)</sub>       |
+| `$gt`      | ✖️        | ✖️       | ✖️                      | ✖️        | ✖️        | ✖️          | ✖️         | ✖️      | ✖️      | ✖️       | ✖️         | ✖️           | ✖️                      |
+| `$gte`     | ✖️        | ✖️       | ✖️                      | ✖️        | ✖️        | ✖️          | ✖️         | ✖️      | ✖️      | ✖️       | ✖️         | ✖️           | ✖️                      |
+| `$lt`      | ✖️        | ✖️       | ✖️                      | ✖️        | ✖️        | ✖️          | ✖️         | ✖️      | ✖️      | ✖️       | ✖️         | ✖️           | ✖️                      |
+| `$lte`     | ✖️        | ✖️       | ✖️                      | ✖️        | ✖️        | ✖️          | ✖️         | ✖️      | ✖️      | ✖️       | ✖️         | ✖️           | ✖️                      |
+| `$in`      | ✖️        | ✖️       | ✖️                      | ✖️        | ✖️        | ✖️          | ✖️         | ✖️      | ✖️      | ✖️       | ✖️         | ✖️           | ✖️                      |
+| `$ne`      | ✖️        | ✖️       | ⚠️ <sub>[[1]](#1)</sub>       | ✅       | ✖️        | ✅         | ✅        | ✅     | ✖️      | ✖️       | ✅        | ✖️           | ⚠️ <sub>[[1]](#1)</sub>       |
+| `$nin`     | ✖️        | ✖️       | ✖️                      | ✖️        | ✖️        | ✖️          | ✖️         | ✖️      | ✖️      | ✖️       | ✖️         | ✖️           | ✖️                      |
+
+
+###### [1] {#1}
+Numbers outside the range of the safe IEEE 754 precision (`< -9007199254740991.0, 9007199254740991.0 >`), 
+will prefetch all numbers larger/smaller than max/min value of the range.