pawanmalwal commented on a change in pull request #3520: [WIP]add spatial-index user guid to doc URL: https://github.com/apache/carbondata/pull/3520#discussion_r364160576
########## File path: docs/spatial-index-guide.md ########## @@ -0,0 +1,94 @@ +<!-- + Licensed to the Apache Software Foundation (ASF) under one or more + contributor license agreements. See the NOTICE file distributed with + this work for additional information regarding copyright ownership. + The ASF licenses this file to you under the Apache License, Version 2.0 + (the "License"); you may not use this file except in compliance with + the License. You may obtain a copy of the License at + + http://www.apache.org/licenses/LICENSE-2.0 + + Unless required by applicable law or agreed to in writing, software + distributed under the License is distributed on an "AS IS" BASIS, + WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. + See the License for the specific language governing permissions and + limitations under the License. +--> + +## What is spatial index + +A spatial index is a data structure that allows for accessing a spatial object efficiently. It is a common technique used by spatial databases. Without indexing, any search for a feature would require a "sequential scan" of every record in the database, resulting in much longer processing time. In a spatial index construction process, the minimum bounding rectangle serves as an object approximation. Various types of spatial indices across commercial and open-source databases yield measurable performance differences. Spatial indexing techniques are playing a central role in time-critical applications and the manipulation of spatial big data. + + + +## What does carbondata implement spatial index + +There are many components that implement spatial indexing, like GeoSpark that use GeoMesa format for spatial query. now carbondata implements a different way of spatial index, more like an UDF. Its core is to use grid coordinates to generate coordinate based hash ID, like Z order, it's also regionally continuous. + +CarbonData implements a grid spatial index. It requires that the data has been gridded when it is load into segments. A set of latitude and longitude represents a grid range, the size of the grid can be specified artificially. So the coordinates of the loaded points are often discrete and not continuous. + +The grid and point relationship is like that black point is the middle of a grid, the red dot is just inside the grid. The red point is inside the grid, it can be replaced by the center point of the grid, indicating that the point is within the grid. Therefore, the coordinates of points in a grid are replaced by black points in the middle. This is the characteristic of data load. At the same time of data load, carbondata will generate hash ID according to the coordinates of rows and columns of the grid. These hash IDs are the same as Z order when querying. Detailed conversion algorithm can refer to the design documents of spatial index. + + + +When querying, the user enters the true space polygon coordinates, carbondata use the polygon and spatial region information passed in when creating a table build a quad tree. The nodes in the quad tree are composed of hash ids generated by the row and column information projected in the polygon area and group photo in map area. When the query polygon area is not disjon from the grid center point, the grid is considered selected. In the following figure, user select a quadrilateral polygon, The grid with the center point in the region will generate a quadtree. A list of line with continuous properties will be generated in the query process, like [97->97 99->99 102->102 104->111 120->120 122->123 151->151 157->158 159->159 192->208 210->210 216->216 225->225 228->229], each part of the list represents a continuous grid area. Carbondata use that line list to prune and filtered. About the detail can be search under https://issues.apache.org/jira/browse/CARBONDATA-3548 Review comment: The nodes in the quad tree are composed of hash ids generated by the row and column information projected in the polygon area and **group photo** in map area. ---------------------------------------------------------------- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. For queries about this service, please contact Infrastructure at: us...@infra.apache.org With regards, Apache Git Services
