Wednesday, October 13, 2021

Compatible with MySQL or Postgres?

Open and closed source scale-out DBMS that are compatible with MySQL and Postgres have emerged on the market. This is great for the community but there will be much confusion about the meaning of compatible

This post has yet to have anything on the cloud vendors in China. They are doing impressive work but I don't know enough about it. I am happy to update this post when I learn more.

But first, the MySQL and Postgres teams.

One way to describe compatibility is via three levels: protocol, syntax and semantics. By upstream I mean MySQL and PostgreSQL.

  • protocol - an app can use existing client libraries to authenticate and connect 
  • syntax - the DBMS will parse SQL that upstream parses. It is not guaranteed to provide semantics that matches upstream and some syntax can be (or might be) parsed but ignored. Syntax compatible implies a best effort to match upstream semantics but that isn't guaranteed nor must it be guaranteed to be useful. 
  • semantics - the DBMS will match upstream semantics. This implies syntax compatible.
Note that syntax and semantics compatibility aren't all or nothing. A syntax compatible DBMS can be useful without supporting (parsing) 100% of the upstream syntax. A semantics compatible DBMS can be useful without supporting or matching behavior for 100% of upstream syntax.

Also note that semantics compatible implies syntax compatible. But protocol compatible implies neither.

Elsewhere in DBMS land

While this post is about MySQL and PostgreSQL, compatibility is growing in popularity elsewhere:
  • MariaDB provides an Oracle compatible mode that provides syntax but not protocol or semantics compatibility. 
  • EnterpriseDB provides an Oracle compatibility product that I don't know much about. 
  • Amazon will soon open source Babelfish that is protocol compatible with SQL Server.
  • Amazon DocumentDB is protocol, syntax and semantics compatible with (an older version of) MongoDB. It supports some (much) of the MongoDB 4.0 API as of October, 2021 per Wikipedia. Public statements suggest this was built on top of, or reusing, the Aurora PostgreSQL code.
Updates
  • added TimescaleDB to Team Postgres
  • added Redshift to Team Postgres
  • added SingleStore to Team MySQL
  • added ClickHouse to Team MySQL
  • added CrateDB to Team Postgres
  • added Dolt to Team MySQL
  • added Team MongoDB
  • added YellowBrick and Greenplum to Team Postgres
  • added Materialize to Team Postgres
  • Building a DBMS to be compatible with MySQL costs more than for Postgres. The Team Postgres projects can reuse BSD licensed PG code while the Team MySQL projects would have to respect the GPL.
  • I have a vague memory of this but to be JDBC compliant the MySQL JDBC driver does a few queries at connect time (either via tables or session/global variables). My JDBC-related bugs are here.

8 comments:

  1. I have been describing it as 3 levels as well! The only differences is naming, where I have been using s/semantics/quirks/

    Measuring compatibility with protocol and syntax is relatively straight forward, but semantics is difficult. Some behaviors are undefined, others are documented.

    One of my favorite examples of a MySQL semantic is that ORDER BY LIMIT 1 and MIN() produce different results. This is documented: https://dev.mysql.com/doc/refman/8.0/en/aggregate-functions.html#function_min

    It a project cares about protocol+syntax only, they might not consider this a bug. But if they care about semantics, it is reasonable to expect an application might depend on this and it should be fixed. For TiDB it was https://github.com/pingcap/tidb/issues/21451

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    Replies
    1. Semantic compatibility will be a big deal for TiDB

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  2. "Team MySQL" also has SingleStore (used to be called MemSQL)

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  3. There's another database emerging as a compatibility standard: ClickHouse. Like PostgreSQL we see people beginning to fork it to create their own analytic solutions. Hydrolix and ByteDance have both done this. There will be more!

    ReplyDelete
    Replies
    1. emerging -> has emerged. ClickHouse is already a big deal.

      Delete
  4. "As the owner of application X, I want to run my application, which was written for MySQL, against your product, which is not MySQL."

    The outcome of that varies for different values of X. But in the end, this is what counts.

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