Query DSL（Domain Specific Language）and SQL（Structured Query Language）are both languages used for querying data, but they have some differences in design and usage scenarios. Here are some of the main differences between them:

1. Origin and Design Purpose:
   • SQL：is a general-purpose query language, widely used in relational database management systems (RDBMS). It has a standard syntax and functionality, used for managing structured data.
   • Query DSL：is a query language specific to Elasticsearch, used for executing search and aggregation operations in Elasticsearch. It is a language designed specifically for search engines, with flexible querying capabilities and support for unstructured data.
2. Data Model:
   • SQL：is suitable for relational data models, where data is stored in tables, and tables are related to each other through foreign keys.
   • Query DSL：is suitable for Elasticsearch's document-field data model, where each document is a self-contained data unit, similar to a JSON object.
3. Query Capability:
   • SQL：provides rich query functionality, including data filtering, sorting, grouping, joining, and subqueries.
   • Query DSL：provides flexible full-text search capabilities, including fuzzy matching, phrase search, range queries, and aggregation analysis. It also supports complex Boolean queries and filtering.
4. Syntax and Structure:
   • SQL：has a fixed syntax structure, such as SELECT, FROM, WHERE, JOIN, etc.
   • Query DSL：is based on JSON format, using key-value pairs to represent query conditions and parameters.
5. Usage Scenario:
   • SQL：is suitable for scenarios that require complex relational data operations, such as transaction processing, multi-table joins, etc.
   • Query DSL：is suitable for scenarios that require fast full-text search and real-time analysis, such as log analysis, e-commerce search, etc.
6. Extensibility and Flexibility:
   • SQL：although it has broad standard support, it may not be flexible enough in handling unstructured data and full-text search.
   • Query DSL：is designed specifically for search engines and can easily be extended to support new query types and analysis functions.

Starting from Elasticsearch 8.x, security authentication is enabled by default, and curl cannot be used to access it directly. You need to use a username and password for authentication. To disable security authentication:

sudo vi /etc/elasticsearch/elasticsearch.yml
xpack.security.enabled: false
sudo systemctl restart elasticsearch

• Forward Index is the most traditional, indexing by ID. However, when searching for a term, it must first retrieve each document one by one and then check if the document contains the required term. This is a process of finding terms based on documents.
• Inverted Index, on the other hand, first finds the term the user wants to search for, gets the IDs of the documents containing that term, and then retrieves the documents based on those IDs. This is a process of finding documents based on terms.