Feed: Planet MySQL.
Author: Gabriela D’Ávila.

This post is a case study of a job I had to do in a legacy application, it doesn’t mean it will apply to you, but it might.

This is a table of contents:

There are many ways to store a tree in a relational database, this is not by far the best option to do it, however it is still common to see it happen.

One way it is called materialized path, which consist with values separated by a delimiter, in my case a comma ,.

You would have a tree stored in a manner like this:

The column asc_path stands for ascending path of a three in where which node has two other ones, not necessarily being a binary three, being stored in the database.

This column has commas in the beginning and in the end because how queries are made to search if an element is present in the path or not by using LIKE "%,id,%". If someone did a search to know if the number 2 was a node in any of the paths, without the commas, it would also return 23, 62 and any other number containing 2.

Performance

The only way to make it a bit faster is having a FULLTEXT index created in asc_path. Because a BTREE index starts indexing in the beginning of a string, since the presence of the wildcard % in the string search it makes it in possible to use said index.

This is the graphical representation of the example above:

Searching

To search an specific element the query would be:

SELECT
parent_id,
user_id,
depth,
asc_path,
node
FROM tree
WHERE asc_path LIKE '%,13,%';

Result:

Converting to a JSON array

Some databases, like PostgresSQL (section 9.42) have more modifiers functions to convert strings to JSON, in my case I wanted to store the ascending tree path in a JSON field which would give me the possibility of using JSON_CONTAINS(json_doc, val) to know the records that have a given node in its path.

To do it, I had to transform the string in a JSON array.

1st step: remove the leading commas

Removing the leading commas, but before any update, lets test what we are doing:

SELECT
parent_id,
user_id,
depth,
asc_path,
TRIM(BOTH ',' FROM asc_path) AS trimmed_commas
FROM tree

Results:

2nd step: add brackets to the string

A JSON array is formed around brackets [], and we need to have it in our string to be a valid JSON document:

SELECT
parent_id,
user_id,
depth,
asc_path,
TRIM(BOTH ',' FROM asc_path) AS trimmed_commas,
CONCAT("[", TRIM(BOTH ',' FROM asc_path), "]") AS added_brackets
FROM tree;

Results:

3rd step: validate if the changes works

Let’s use JSON_VALID() to see if it will accept our new string as a JSON, keep in mind that when the argument is NULL the return is also NULL:

SELECT
parent_id,
user_id,
depth,
asc_path,
TRIM(BOTH ',' FROM asc_path) AS trimmed_commas,
CONCAT("[", TRIM(BOTH ',' FROM asc_path), "]") AS added_brackets,
JSON_VALID(CONCAT("[", TRIM(BOTH ',' FROM asc_path), "]")) AS json_valid
FROM tree;

Results:

Replacing 1st step and 2nd step with a function

So that your query gets easier to use and not messy, you can create a function, I decided to create to_json_array(input_string, delimiter_char):

Running the query only with to_json_array on MySQL:

SELECT
parent_id,
user_id,
depth,
asc_path,
to_json_array(asc_path, ',') AS to_json_array,
JSON_VALID(to_json_array(asc_path, ',')) AS is_to_json_array_valid,
node
FROM tree;

Result:

Disclaimer

This function is not native, and its use in production is not guaranteed.

Notice that the database returns the JSON as valid making it possible to convert that TEXT to a new column asc_path_json:

ALTER TABLE tree
ADD COLUMN asc_path_json JSON
AFTER asc_path;

UPDATE tree
SET asc_path_json = to_json_array(asc_path, ',');

Which gives us the ability to check more quickly if an item is in the path for that node:

SELECT *
FROM tree
WHERE json_contains(asc_path_json, "13");

Result: