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Postgres 15 improves UNIQUE and NULL

By Ryan Lambert -- Published July 11, 2022

Postgres 15 beta 2 was released recently! I enjoy Beta season... reviewing and testing new features is a fun diversion from daily tasks. This post takes a look at an improvement to UNIQUE constraints on columns with NULL values. While the nuances of unique constraints are not as flashy as making sorts faster (that's exciting!), improving the database developer's control over data quality is always a good benefit.

This email chain has the history behind this change. The Postgres 15 release notes summarize this improvement:

"Allow unique constraints and indexes to treat NULL values as not distinct (Peter Eisentraut)

Previously NULL values were always indexed as distinct values, but this can now be changed by creating constraints and indexes using UNIQUE NULLS NOT DISTINCT."

Two styles of UNIQUE

To take a look at what this change does, we create two tables. The null_old_style table has a 2-column UNIQUE constraint on (val1, val2). The val2 allows NULL values.

CREATE TABLE null_old_style
(
    id BIGINT GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY,
    val1 TEXT NOT NULL,
    val2 TEXT NULL,
    CONSTRAINT uq_val1_val2
        UNIQUE (val1, val2)
);

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H3 indexes for performance with PostGIS data

By Ryan Lambert -- Published June 24, 2022

I recently started using the H3 hex grid extension in Postgres with the goal of making some not-so-fast queries faster. My previous post, Using Uber's H3 hex grid in PostGIS, has an introduction to the H3 extension. The focus in that post, admittedly, is a PostGIS focused view instead of an H3 focused view. This post takes a closer look at using the H3 extension to enhance performance of spatial searches.

The two common spatial query patterns considered in this post are:

  • Nearest neighbor style searches
  • Regional analysis

Setup and Point of Focus

This post uses two tables to examine performance. The following queries add an h3_ix column to the osm.natural_point and osm.building_polygon tables. This approach uses GENERATED columns and adds an index to the column. Going through these steps allow us to remove the need for PostGIS joins at query time for rough distance searches. See my previous post for details about installing the H3 extension and the basics of how it works.

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Using Uber's H3 hex grid in PostGIS

By Ryan Lambert -- Published April 24, 2022

This post explores using the H3 hex grid system within PostGIS. H3 was developed by Uber and has some cool benefits over the PostGIS native ST_HexagonGrid() function used in my post Find missing crossings in OpenStreetMap with PostGIS. The hex grid built-in to PostGIS is great for one-off projects covering a specific region, though it has shortcomings for larger scale consistency. On the other hand, the H3 grid is a globally defined grid that scales up and down through resolutions neatly. For more details, read Uber's description.

This post works through a few of the functions available in the H3 extension and how they can be used for spatial aggregation in an analysis. One additional focus is how to generate a table of H3 hexagons for a given resolution.

Note: This post does not focus on using H3 for the best performance. See my post H3 indexes for performance with PostGIS data for a look into high performance spatial searches with H3.

Install H3 in Postgres

The H3 library is available to PostGIS as a Postgres extension. I am using the bytesandbrains h3-pg project available on GitHub. The extension can be installed using pgxn install h3. Once installed, create the H3 extension in the database.

CREATE EXTESION h3;

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Postgres Data Dictionary for everyone

By Ryan Lambert -- Published January 04, 2022

A data dictionary is an important tool for anyone that stores and consumes data. The PgDD extension makes it easy to inspect and explore your data structures in Postgres. This post shows how PgDD provides access to current and accurate information about your databases for a variety of users:

  • Analysts
  • DBAs and Developers
  • The Business

This data dictionary information from PgDD is made available using standard SQL by querying a small set of views.

Background

Relational databases, including Postgres, track the majority of the information needed for a data dictionary. This is done in the underlying system catalogs; Postgres' system catalogs are in the pg_catalog schema. The challenge with using the system catalogs is they are not very user friendly to query for the type of details commonly needed. PgDD does not do anything magical, it is simply a wrapper around the Postgres system catalogs!

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Permissions required for PostGIS

By Ryan Lambert -- Published December 01, 2021

PostGIS is a widely popular spatial database extension for Postgres. It's also one of my favorite tools! A recent discussion on the People, Postgres, Data Discord server highlighted that the permissions required for various PostGIS operations were not clearly explained in the PostGIS documentation. As it turned out, I didn't know exactly what was required either. The basic PostGIS install page provides resources for installing the binary on the server and the basic CREATE EXTENSION commands, but does not explain permissions required.

This post explores the permissions required for three types of PostGIS interactions:

  • Install/Create PostGIS
  • Use PostGIS
  • Load data from pg_dump

Database and Users

I am using Postgres installed on my laptop for these tests, Postgres 13.5 and PostGIS 3.1. I created an empty database named postgis_perms and check the \du slash command in psql to see the current roles. This instance has my my ryanlambert role, a superuser, and the default postgres role. The postgres role is not used in this post outside of this example.

([local] 🐘) ryanlambert@postgis_perms=# \du
                                     List of roles
┌─────────────┬────────────────────────────────────────────────────────────┬───────────┐
│  Role name  │                         Attributes                         │ Member of │
╞═════════════╪════════════════════════════════════════════════════════════╪═══════════╡
│ postgres    │ Superuser, Create role, Create DB, Replication, Bypass RLS │ {}        │
│ ryanlambert │ Superuser, Create role, Create DB                          │ {}        │
└─────────────┴────────────────────────────────────────────────────────────┴───────────┘

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