This document provides a high-level overview of the various objects and primitives that make up the PostHog data model. For more information on exactly how data is stored in our database, check out the ClickHouse overview.
The two most-basic entities in PostHog are the Event and Person objects, and represent the core of our analytics functionalities.
Event
An event is the most important object in PostHog, and represents a single action that a user performed at a specific point in time. These events are sent either from one of our libraries or directly via our API.
Each event contains the following base fields within ClickHouse:
| Column | Type | Description |
|---|---|---|
| uuid | UUID | ID of the event |
| team_id | Int64 | Foreign key which links to the team |
| event | VARCHAR | Name of the event |
| distinct_id | VARCHAR | The unique or anonymous id of the user that triggered the event. |
| properties | VARCHAR | Any key: value pairs in a dict. - $current_url - we use this in a couple of places (like /paths, /events) as the url the user was visiting at that time. |
| elements_* | Various | Columns used for $autocapture to track which DOM element was clicked on |
| timestamp | DateTime64(6, 'UTC') | Defaults to timezone.now at ingestion time if not set |
| created_at | DateTime64(6, 'UTC') | The timestamp for when the event was ingested |
| person_id | UUID | This is the id of the Person that sent this event |
| person_created_at | DateTime64(3) | The timestamp of the earliest event associated with this person |
| person_properties | VARCHAR | A JSON object with all the properties for a user, which can be altered using the $set, $set_once, and $unset arguments |
| group* | Various | Columns used for group analytics |
Events are only stored within ClickHouse, and once they have been written they can't be changed. This limitation comes from a trade-off in the design of ClickHouse: inserting data and running queries on large tables is extremely fast, but updating or deleting specific rows is generally not efficient.
Person
In PostHog, a Person is an entity which sends events, and typically represents a 'User' in most implementations.
Each person contains the following base fields within PostgreSQL:
| Column | Type | Description |
|---|---|---|
| id | integer | Sequential ID for the person |
| team_id | integer | Foreign key which links to the team |
| uuid | UUID | UUID of the person within ClickHouse. This is referenced by the person_id field on events |
| created_at | timestamptz | The timestamp of the earliest event associated with this person |
| properties | jsonb | A JSON object with all the properties for a user, which can be altered using the $set, $set_once, and $unset arguments |
| version | bigint | Incremented every time a person is updated. Helps to keep ClickHouse and PostgreSQL in sync. |
Persons are stored in PostgreSQL, but are additionally replicated into ClickHouse for certain queries. For example, when viewing the global list of Persons from the dashboard, this information is retrieved from ClickHouse.
Additionally, person properties are also stored directly on each event. Their value is determined during ingestion by looking up the Person who sent the event in PostgreSQL, and combining these values with any updates from the event itself.
The properties field on each Person object can be updated at any time, and as a result the PostgreSQL table represents the one source of truth for the most up-to-date values for the properties of a Person.
For more information on how person properties are added to events, take a look at this step in the ingestion overview.