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Posted to oak-commits@jackrabbit.apache.org by jo...@apache.org on 2022/07/05 07:57:50 UTC
[jackrabbit-oak] branch trunk updated: describe the process of executing a query (#608)
This is an automated email from the ASF dual-hosted git repository.
joerghoh pushed a commit to branch trunk
in repository https://gitbox.apache.org/repos/asf/jackrabbit-oak.git
The following commit(s) were added to refs/heads/trunk by this push:
new 7aee0b1c26 describe the process of executing a query (#608)
7aee0b1c26 is described below
commit 7aee0b1c264476097c2ecd4499fdb01e6c6eae6b
Author: Jörg Hoh <jo...@users.noreply.github.com>
AuthorDate: Tue Jul 5 09:57:45 2022 +0200
describe the process of executing a query (#608)
describe the query execution on a high level.
Co-authored-by: Dan Klco <kl...@users.noreply.github.com>
Co-authored-by: Fabrizio Fortino <fa...@gmail.com>
Co-authored-by: Dan Klco <kl...@users.noreply.github.com>
---
oak-doc/src/site/markdown/query/query-engine.md | 82 ++++++++++++++++---------
1 file changed, 52 insertions(+), 30 deletions(-)
diff --git a/oak-doc/src/site/markdown/query/query-engine.md b/oak-doc/src/site/markdown/query/query-engine.md
index 1ed3ec629e..ce662539a5 100644
--- a/oak-doc/src/site/markdown/query/query-engine.md
+++ b/oak-doc/src/site/markdown/query/query-engine.md
@@ -22,36 +22,41 @@ grep "^#.*$" src/site/markdown/query/query-engine.md | sed 's/#/ /g' | sed 's
## The Query Engine
-* [Overview](#Overview)
- * [Query Processing](#Query_Processing)
- * [Cost Calculation](#Cost_Calculation)
- * [Query Options](#Query_Options)
- * [Query Option Traversal](#Query_Option_Traversal)
- * [Query Option Index Tag](#Query_Option_Index_Tag)
- * [Query Option Offset / Limit](#Query_Option_Offset__Limit)
- * [Index Selection Policy](#Index_Selection_Policy)
- * [Compatibility](#Compatibility)
- * [Result Size](#Result_Size)
- * [Quoting](#Quoting)
- * [Equality for Path Constraints](#Equality_for_Path_Constraints)
- * [Slow Queries and Read Limits](#Slow_Queries_and_Read_Limits)
- * [Keyset Pagination](#Keyset_Pagination)
- * [Full-Text Queries](#Full-Text_Queries)
- * [Excerpts and Highlighting](#Excerpts_and_Highlighting)
- * [Native Queries](#Native_Queries)
- * [Similarity Queries](#Similarity_Queries)
- * [Spellchecking](#Spellchecking)
- * [Suggestions](#Suggestions)
- * [Facets](#Facets)
- * [XPath to SQL-2 Transformation](#XPath_to_SQL-2_Transformation)
- * [The Node Type Index](#The_Node_Type_Index)
- * [Temporarily Disabling an Index](#Temporarily_Disabling_an_Index)
- * [Deprecating Indexes](#Deprecating_Indexes)
- * [The Deprecated Ordered Index](#The_Deprecated_Ordered_Index)
- * [Index Storage and Manual Inspection](#Index_Storage_and_Manual_Inspection)
- * [SQL-2 Optimisation](#SQL-2_Optimisation)
- * [Additional XPath and SQL-2 Features](#Additional_XPath_and_SQL-2_Features)
- * [Temporarily Blocking Queries](#Temporarily_Blocking_Queries)
+- [The Query Engine](#the-query-engine)
+- [Overview](#overview)
+ - [Query Processing](#query-processing)
+ - [Cost Calculation](#cost-calculation)
+ - [Identifying Nodes](#identifying-nodes)
+ - [Ordering](#ordering)
+ - [Iterating the Result Set](#iterating-the-result-set)
+ - [Query Options](#query-options)
+ - [Query Option Traversal](#query-option-traversal)
+ - [Query Option Offset / Limit](#query-option-offset--limit)
+ - [Query Option Index Tag](#query-option-index-tag)
+ - [Index Selection Policy](#index-selection-policy)
+ - [Compatibility](#compatibility)
+ - [Result Size](#result-size)
+ - [Quoting](#quoting)
+ - [Equality for Path Constraints](#equality-for-path-constraints)
+ - [Slow Queries and Read Limits](#slow-queries-and-read-limits)
+ - [Keyset Pagination](#keyset-pagination)
+ - [Full-Text Queries](#full-text-queries)
+ - [Excerpts and Highlighting](#excerpts-and-highlighting)
+ - [SimpleExcerptProvider](#simpleexcerptprovider)
+ - [Native Queries](#native-queries)
+ - [Similarity Queries](#similarity-queries)
+ - [Spellchecking](#spellchecking)
+ - [Suggestions](#suggestions)
+ - [Facets](#facets)
+ - [XPath to SQL-2 Transformation](#xpath-to-sql-2-transformation)
+ - [The Node Type Index](#the-node-type-index)
+ - [Temporarily Disabling an Index](#temporarily-disabling-an-index)
+ - [Deprecating Indexes](#deprecating-indexes)
+ - [The Deprecated Ordered Index](#the-deprecated-ordered-index)
+ - [Index Storage and Manual Inspection](#index-storage-and-manual-inspection)
+ - [SQL-2 Optimisation](#sql-2-optimisation)
+ - [Additional XPath and SQL-2 Features](#additional-xpath-and-sql-2-features)
+ - [Temporarily Blocking Queries](#temporarily-blocking-queries)
## Overview
@@ -122,6 +127,23 @@ so the method should be reasonably fast (not read any data itself, or at least n
If an index implementation can not query the data, it has to return `Infinity` (`Double.POSITIVE_INFINITY`).
+#### Identifying Nodes
+
+If an index is selected, the query is executed against the index. The translation from the JCR Query syntax into the query language supported by the index includes as many constraints as possible which are supported by the index. Depending on the index definition this can mean that not all constraints can be resolved by the index itself.
+In this case, the Query Engine tries to let the index handle as much constraints as possible and later executes all remaining constraints while [Iterating the Result Set](#iterating-the-result-set) by retrieving the nodes from the node store and evaluating the nodes against the constraints. Each retrieval, including non-matching nodes is counted as a traversal. This means that despite the use of an index an additional traversal can be required if not all constraints in a query are execut [...]
+
+If no matching index is determined in the previous step, the Query Engine executes this query solely based on a traversing the Node Store and evaluating the nodes against the constraints.
+
+#### Ordering
+If a query requests an ordered result set, the Query Engine tries to get an already ordered result from the index; in case the index definition does not support the requested ordering or in case of a traversal, the Query Engine must execute the ordering itself. To achieve this the entire result set is read into memory and then sorted. This consumes memory, takes time and requires the Query Engine to read the full result set even in the case where a limit setting would otherwise limit the [...]
+
+#### Iterating the Result Set
+Query results are implemented as lazy iterators, and the result set is only read if needed. When the next result is requested, the result iterator seeks the potential nodes to find the next node matching the query.
+During this seek process the Query Engine reads and filters the potential nodes until it finds a match. Even if the query is handled completely by an index, the Query Engine needs to check if the requesting session is allowed to read the nodes.
+
+That means that during this final step every potential node must be loaded from the node store, thus counting towards the read limit (see [Slow Queries and Read Limits](#slow-queries-and-read-limits)).
+
+
### Query Options
With query options, you can enforce the usage of indexes (failing the query if there is no index),