Hacker News Reader

Comment by captaintobs | Hacker News Reader

Author here, feel free to ask me any questions!

Something that I'm working on is a pure python SQL engine https://github.com/tobymao/sqlglot/blob/main/sqlglot/executo.... It does the whole shebang, parsing, optimizations, logical planning, physical execution.

View on HN

Holy smokes, this is super impressive!

I have a personal question if you don't mind -- I do some SQL query generation and transpilation for both work and hobby. One headache I've run into recently is generating nested EXISTS() subqueries.

Imagine you have something like this:

    // WHERE Name = 'Audioslave'
    {
        type: "binary_op",
        operator: "equal",
        column: { path: [], name: "Name" },
        value: { type: "scalar", value: "Audioslave" },
    }

This is all fine, but what if you want to say "A binary operation on related entities":

    {
        type: "binary_op",
        operator: "equal",
        column: { path: ["Albums", "Tracks"], name: "AlbumId" },
        value: { type: "scalar", value: 40 },
    },

Which you want to generate something like:

  WHERE EXISTS(SELECT 1 FROM Albums WHERE Albums.ForeignKey = t.PrimaryKey
  AND EXISTS(SELECT 1 FROM Tracks WHERE Tracks.ForeignKey = Albums.PrimaryKey AND AlbumId = 40))

How to do this is giving me a headache for a lot of reasons and I can't seem to come up with a good way. Any tips, references, or search terms to google?

Thank you, look forward to digging in more + gave your repo a star!

i rarely ever use exists and prefer to do left joins or left semi joins (in spark)

i'm not exactly sure what you're asking though, in terms of sql generation, it's not difficult for me because i just take in sql and output sql from the ast

Exists can be very efficient as it allows execution to stop immediately when something is found.

Any plans on supporting snowflake? May I submit a PR?

Id love to support more engines. PRs are very welcome!

Wow this looks neat. I've been looking for an easy to use sql parser (I've tried sqlparse).

How permissive is the parser? Do you need to know the exact dialect up front or can it make some intelligent guesses for unknown UDFs etc?

The parser is very permissive. You don’t need the exact dialect upfront, it handles unknown udfs internally as anonymous funcs.

Thanks, I'll definitely have a look.

Optimisation is a tricky one as you need the various cardinalities, how do you handle that?

I don’t do join order optimizations because that relies on cardinality estimation, I leave that up to the physical plan. But there’s plenty more to optimize like predicate and projection pushdown.

Hey, great work! Can you talk a bit about the use case that inspire you to write the tool?

At the large tech companies I've been working at, there are many different big data engines that speak different dialects of sql (presto / spark). People write SQL queries in one language and want to run it in another, but it doesn't just work, there are many parts of the query that need to be manually changed in order for it to run which is tedious and error prone.

Where I work, we handle it by having Data Scientists running experiments on our platform commit their queries as Python code to a repository of metrics.

I actually built the system you work on :). It was the main inspiration for this project because PyPika is not a great experience for data scientists.

Thought you might catch that! I've actually helped swap a few things to SQLGlot from pypika.

Got it. I'm asking because I worked on something similar. The idea was to unit test some Airflow workflows locally. The production workflows were using Hive, but having a local Hive container was too slow for tests, so we wrote a small parser to translate the Hive queries into SQLite queries at runtime. In the end, we had a decent PoC but couldn't complete it because of all the Hive features, but it was super fun.

yep, we do the same but using duckdb, it's got a lot more analytical functions and closer to hive than sqlite

Reply on news.ycombinator.com