Designing a PostgreSQL Document API

August 20 2015 Fun Nosql Postgres

PostgreSQL as many know, supports JSON as a storage type and with the release of 9.4, Postgres now supports storing JSON as jsonb - a binary format.

This is great news for people who want to move beyond simple "store JSON as text". jsonb supports indexing now using the GIN index, and also has a special query operator that takes advantage of the GIN index.

Who Cares?

It's been fun to explore jsonb in Postgres and to see what's possible. Which is kind of the problem: it's only an exploration and some musing, to get any work done leaves a little to be desired.

What I mean is that other systems (like RethinkDB) have a ton of functionality already built in to help you save documents, query documents, and optimize things. Postgres has some interesting abilities this way - but out of the box querying is pretty ... lacking to be honest.

Consider this query:

select document_field -> 'my_key' from my_docs
where document_field @> '{"some_key" : "some_value"}';

It surfaces a bit of weirdness when it comes to JSON and Postgres: it's all strings. Obviously SQL has no understanding of JSON, so you have to format it as a string. Which means working directly with JSON in SQL is a pain. Of course if you have a good query tool that problem is lessened to a degree... but it still exists.

In addition, the storage of a document is a little free-for-all. Do you have a single column that's jsonb? Or Multiple columns in a larger table structure? It's up to you - which is nice but too many choices can also be paralyzing.

So why worry about all of this? If you want to use a document database then use a document database. I agree with that... but there's one really compelling reason to use Postgres (for me at least)...

iu

Postgres is ACID-compliant. That means you can rely on it to write your data and, hopefully, not lose it.

Postgres is also relational, which means that if you want to graduate to a stricter schema as time goes on you can. There are a number of reasons you might want to choose Postgres - for now let's say you have made that choice and want to start working with Documents and jsonb.

A Better API

Personally, I'd love to see more functions that support the notion of working with documents. Right now we have built-ins that support working with the JSON types - but nothing that supports a higher level of abstraction.

That doesn't mean we can't build such an API ourselves. Which I did :). Here goes...

A Document Table

I want to store documents in a table that has some meta information as well as additional ways I can query the information, specifically: Full Text Search.

The structure of the table can be opinionated - why not we're building out this abstraction! Let's start with this:

create table my_docs(
  id serial primary key,
  body jsonb not null,
  search tsvector,
  created_at timestamptz not null default now(),
  updated_at timestamptz not null default now()
)

There will be some duplication here. The document itself will be stored in the body field, including the id, which is also stored as a primary key (we need this because this is still Postgres). I'm embracing duplication, however, because:

Saving a Document

What I'd like in a save_document function is the ability to...

I can do this by creating my own function save_document and, for fun I'll use PLV8 - Javascript in the database. In fact I'll create two functions - one that specifically creates my table, and another that saves the document itself.

First, create_document_table:

create function create_document_table(name varchar, out boolean)
as $$
  var sql = "create table " + name + "(" +
    "id serial primary key," +
    "body jsonb not null," +
    "search tsvector," +
    "created_at timestamptz default now() not null," +
    "updated_at timestamptz default now() not null);";

  plv8.execute(sql);
  plv8.execute("create index idx_" + name + " on docs using GIN(body jsonb_path_ops)");
  plv8.execute("create index idx_" + name + "_search on docs using GIN(search)");
  return true;
$$ language plv8;

This function creates a table and appropriate indexes - one for the jsonb field in our document table, the other for the tsvector full text index. You'll notice that I'm building SQL strings on the fly and executing with plv8 - that's the way you do it with Javascript in Postgres.

Next, let's create our save_document function:

create function save_document(tbl varchar, doc_string jsonb)
returns jsonb
as $$
  var doc = JSON.parse(doc_string);
  var result = null;
  var id = doc.id;
  var exists = plv8.execute("select table_name from information_schema.tables where table_name = $1", tbl)[0];

  if(!exists){
    plv8.execute("select create_document_table('" + tbl + "');");
  }

  if(id){
    result = plv8.execute("update " + tbl + " set body=$1, updated_at = now() where id=$2 returning *;",doc_string,id);
  }else{
    result = plv8.execute("insert into " + tbl + "(body) values($1) returning *;", doc_string);
    id = result[0].id;
    doc.id = id;
    result = plv8.execute("update " + tbl + " set body=$1 where id=$2 returning *",JSON.stringify(doc),id);
  }

  return result[0] ? result[0].body : null;

$$ language plv8;

I'm sure this function looks a bit strange, but if you read through each line you should be able to figure out a few things. But why the JSON.parse() call?

This is because the Postgres jsonb type is not really JSON here - it's a string. Outside our PLV8 bits is still Postgres World and it works with JSON as a string (storing it in jsonb in a binary format). So, when our document is passed to our function it's as a string, which we need to parse if we want to work with it as a JSON object in Javascript.

In the insert clause you'll notice that I have to synchronize the ID of the document with that of the primary key that was just created. A little cumbersome, but it works fine.

Finally - you'll notice that in the original insert call as well as the update, I'm just passing the doc_string argument right into the plv8.execute call as a parameter. That's because you need to treat JSON values as strings in Postgres.

This can be really confusing. If I try to send in doc (our JSON.parsed object) it will get turned into [Object object] by plv8. Which is weird.

Moreover if I try to return a Javascript object from this function (say, our doc variable) - I'll get an error that it's an invalid format for the type JSON. Which is ultra confusing.

For our result I'm simply returning the body from our query result - and it's a string, believe it or not, and I can just pass it straight through as a result. I should note here as well that all results from plv8.execute return an Array of items that you can work with as Javascript objects.

The Result

It works really well! And it's fast. If you want to try it out you'll need to install the PLV8 extension and then write your query accordingly:

create extension plv8;
select * from save_document('test_run', '{"name" : "Test"}');

You should see a new table and a new record in that table:

save_document_1

More To Do

In the next post I'll add some additional features, specifically:

This is a good start!