docs: faq section + post-join filter question (#578)

substrait-io · Dec 5, 2023 · 7629e0e · 7629e0e
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diff --git a/site/docs/_config b/site/docs/_config
@@ -9,3 +9,6 @@ arrange:
   - community
   - governance.md
   - about.md
+  - tools
+  - tutorial
+  - faq.md
diff --git a/site/docs/faq.md b/site/docs/faq.md
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+---
+title: FAQ
+---
+# Frequently Asked Question
+
+## What is the purpose of the post-join filter field on Join relations?
+The post-join filter on the various Join relations is not always equivalent to an explicit Filter relation AFTER the Join.
+
+See the example [here](https://facebookincubator.github.io/velox/develop/joins.html#hash-join-implementation) that highlights how the post-join filter behaves differently than a Filter relation in the case of a left join.