Скачать или смотреть How to Create a PostGIS Update Trigger to Maintain Spatial Relationships in Your Database

Learn how to efficiently update your PostgreSQL databases using PostGIS with a trigger that ensures the relationship between points and polygons remains intact.
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This video is based on the question https://stackoverflow.com/q/71500716/ asked by the user 'Nadiia' ( https://stackoverflow.com/u/2015940/ ) and on the answer https://stackoverflow.com/a/71501954/ provided by the user 'JGH' ( https://stackoverflow.com/u/7635569/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Managing Spatial Relationships with a PostGIS Trigger

In database management, maintaining accurate relationships between spatial data is crucial. Imagine you have a system where points of interest (like GPS coordinates) need to be updated based on newly defined areas (such as geographical polygons). In this guide, we'll discuss how to set up a PostGIS trigger to update a third table whenever a polygon is added, ensuring spatial data integrity in your application.

Understanding the Problem

You have three tables in your PostgreSQL database:

table_1: Contains a list of points identified by point_id with an associated geometry in geometry (Point).

table_2: Holds polygons identified by area_id with corresponding geometry in geometry (Polygon).

table_3: This table consolidates the relationship between point_id and area_id, documenting which points reside within each polygon.

As new polygons are added to table_2, you want table_3 to automatically reflect the points that fall within these new areas.

Setting Up the Trigger Function

To solve this problem, we begin with the following SQL function. This function will insert the appropriate point_id and area_id into table_3 whenever a new polygon is added.

Initial Function Code

[[See Video to Reveal this Text or Code Snippet]]

Creating the Trigger

After defining the function, you create a trigger that calls this function:

[[See Video to Reveal this Text or Code Snippet]]

Addressing the Unique Constraint Error

While the above setup seems logical, you may encounter a unique constraint error when trying to insert duplicate relationships in table_3. The error message typically looks like this:

[[See Video to Reveal this Text or Code Snippet]]

Solution to Avoid Duplicate Entries

Change the SQL Logic: Instead of joining the two tables (which could lead to unwanted cross-joins), simply check each point against the newly added polygon using the NEW object. The updated SQL query is as follows:

[[See Video to Reveal this Text or Code Snippet]]

This change eliminates the need for a join and reduces the likelihood of entering duplicate records when a point falls within the same polygon multiple times.

Implementing a Cleanup Trigger: Additionally, it's essential to set up another trigger to remove entries from table_3 when corresponding entries in either table_1 or table_2 are deleted. This helps keep your data consistent and avoids orphaned records in table_3.

Conclusion

Using triggers effectively can streamline your database operations and ensure that spatial relationships are accurately maintained. By implementing the above strategies to address duplicate constraints, you can keep your table_3 accurate and up-to-date.

Using PostGIS triggers not only simplifies data management but also enhances the overall performance of your spatial queries. With these approaches, you’ll have a robust system to manage your geographical data efficiently.

Feel free to ask questions or share your experiences with PostGIS triggers in the comments below!