From 9cb88a28e0242dd879063215731b90ebce7ae8b8 Mon Sep 17 00:00:00 2001
From: Yury-Fridlyand <yury.fridlyand@improving.com>
Date: Fri, 16 Jun 2023 17:13:06 -0700
Subject: [PATCH] Design doc for `LIMIT` in pagination.

Signed-off-by: Yury-Fridlyand <yury.fridlyand@improving.com>
---
 docs/dev/limit-in-pagination.md | 62 +++++++++++++++++++++++++++++++++
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 create mode 100644 docs/dev/limit-in-pagination.md

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+## Background
+
+Scrolled search is performed by request to `http://localhost:9200/<index>/_search?scroll=<timeout>` endpoint and it has a mandatory field `size`, which defines page size:
+```json
+{
+    "size" : 5
+}
+```
+Regular (non-scrolled/non-paged) search is performed by request to `http://localhost:9200/<index</_search` endpoint and it has not mandatory fields. Parameter `size` though defines maximum number of docs to be returned by search.
+
+## Problem statement
+
+`LIMIT` clause is being converted to `size` by SQL plugin during push down operation.
+Hereby comes the conflict in using `LIMIT` with pagination.
+
+## Solution
+
+Don't do push down for `LIMIT`. `LimitOperator` Physical Plan Tree node will cut off yielding search results with minimal overhead.
+It seems simple, but here comes another problem.
+
+## Impediments
+
+`Optimizer` which calls push down operations sets page size after limit, which overwrites this settings. `Optimizer` does not apply rules in the order they are given.
+
+## Fix
+
+Rework [`Optimizer`](https://github.com/opensearch-project/sql/blob/57ce303740f64fe0279fc34aab0116f33f11fbe6/core/src/main/java/org/opensearch/sql/planner/optimizer/LogicalPlanOptimizer.java).
+Current behavior:
+```py
+def Optimize:
+    for node in PlanTree: # Traverse the Logical Plan Tree
+        for rule in rules: # Enumerate rules
+            tryApplyRule()
+```
+
+Expected behavior:
+```py
+def Optimize:
+    for rule in rules: # Enumerate rules
+        for node in PlanTree: # Traverse the Logical Plan Tree
+            tryApplyRule()
+```
+Rules list:
+```
+...
+CreateTableScanBuilder
+PushDownPageSize
+...
+PUSH_DOWN_LIMIT
+...
+```
+
+This gives us warranty that `pushDownLimit` operation would be rejected if `pushPageSize` called before. Then, not optimized Logical Plan Tree node `LogicalLimit` will be converted to `LimitOperator` Physical Plan tree node.
+
+## Other changes
+
+1. Make `Optimizer` rules applied only once.
+2. Reorder `Optimizer` rules.
+3. Make `LimitOperator` properly serialized and deserialized.
+4. Make `OpenSearchIndexScanBuilder::pushDownLimit` return `false` if `pushDownPageSize` was called before.
+5. (Optional) Groom `Optimizer` to reduce amount of unchecked casts and uses raw classes.
+6. (Optional) Rework `Optimizer` to make it a tree visitor.
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