PostgreSQL Features Missing in Amazon Redshift

Redshift is an Amazon database technology available for AWS customers. It is based on PostgreSQL 8. The codebase was modified to develop a horizontally scalable SQL DBMS, suitable for analytics, and in particular Data WareHouse.

Many PostgreSQL features are not available in Redshift, particularly those that are not suitable for a database cluster or for analytics, and those that Amazon didn’t consider important enough to spend the necessary resources to maintain them.

This article shows an incomplete list of PostgreSQL features that are missing in Redshift. It is mainly written for those who are familiar with PostgreSQL and are evaluating Redshift for analytics.

Main missing features

Let me stress it again: this is not meant to be a complete list.

The code

Redshift is not open source, nor source available.

PostgreSQL is open source, but its license does not disallow Amazon from using its codebase to build a cloud database.

Ownership

One great thing about PostgreSQL is that it is not developer or owned by a company. Instead, many organisations and individuals contribute to improve it and have interest in keeping it alive.

Redshift’s future appears solid because its owner appears solid. Still, it depends on a single company. As a corner case, should Amazon’s plans change for any reason, Redshift may even be retired at some point.

Latency

While Redshift has a good throughput, the latency is far too high and unpredictable for OLTP workloads. Redshift is only intended for analytics.

SELECT *

Redshift stores data in a columnar fashion. As a consequence, selecting all columns (SELECT *) or many columns causes performance penalties.

Extensions

Redshift does not support the concept of extensions.

Data types

Redshift does not support certain types, like:

• ARRAYs;
• range types;
• json and xml;
• bit and bytea;
• network types;
• UUID.

User-defined types, pseudo-types and domains are also not supported.

Indexes

Redshift allows a DISTKEY and a SORTKEY for each table (very similar to Cassandra primary keys), but there are no secondary indexes.

No index type can be specified.

Constraints

Constraints like primary key uniqueness, UNIQUE constraints and foreign keys are not enforced. Instead, Redshift assumes that they are enforced by the user, so for example:

• SELECT DISTINCT pk will return the same results as SELECT pk.
• If there is a primary key or a UNIQUE key, COUNT(DISTINCT *) will return the same number as COUNT(*).

CHECK constraints, ASSERTIONs and triggers are not supported.

Foreign Data Wrappers

Foreign data wrappers are not supported in Redshift.

Though, it has external tables to use data in some specific formats stored in Amazon S3. The UNLOAD statement can be used to export data to Apache Parquet files in S3.

Transactions

Transactions have the following important limitations:

• The only supported isolation level is SERIALIZABLE;
• SAVEPOINTs are not supported;
• TRUNCATE and VACUUM implicitly commit the current transaction.

But again, keep in mind that Redshift is designed for data warehousing. It allows to:

• Load data while users run queries;
• Load data concurrently into different tables;
• Alter a dimension structure and its data inside the same transaction.

SQL

The following statements (between others) are missing:

• MERGE;
• COPY has less options and more specific semantics;
• CHECKPOINT;
• LISTEN;
• SECURITY LABEL;
• VALUES.

External languages

Stored procedures, functions and triggers can only be written in PL/plSQL. Other procedural languages are not supported.

Conclusions

I used Redshift in the past and I will probably use it again. Let’s be clear: as a general rule, I prefer to use open source technologies, and in particular I don’t like to depend on a single vendor. But many organisations are already depending on Amazon, and don’t have any plans to change that.

Redshift is not easy because it’s in the cloud. It’s not something easy to switch to because it’s just good ol’ Postgres. You will need to learn things like how keys work and compression. Its costs are also potentially high (something not covered in this article).

All that said, would we recommend it? Sometimes we would. The main reason for that is integration with other Amazon services, where lock-ins are considered acceptable.

Federico Razzoli

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