Writing HogQL queries in Python

❗️ This guide is intended only for development of PostHog itself. If you're looking for documentation on writing HogQL queries, go to HogQL.

HogQL is our layer on top of ClickHouse SQL which provides nice features such as:

Automatic person/group/etc property joins depending on the team/context
Customisable database schema per team
Flexible AST-powered templating language for building queries.

Query templates

HogQL queries are built up from AST (Abstract Syntax Tree) nodes.

You can build the nodes yourself, or use the helpers parse_expr and parse_select to convert HogQL strings into AST nodes:

from posthog.hogql import ast
from posthog.hogql.query import execute_hogql_query
from posthog.hogql.parser import parse_expr, parse_select

num_last_days = 2

stmt = parse_select(
    "select event, timestamp from events where {where} limit 100", 
    { 
        'where': parse_expr(
            'timestamp > interval {days} days', 
            { 'days': ast.Constant(value=num_last_days) }
        ) 
    }
)

query_result = execute_hogql_query(query=stmt, team=team, query_type="used in logs")
query_result.results == [...]
query_result.columns == ['event', 'timestamp']  # might be useful if you select '*'

Few things to note:

parse_select parses full SELECT queries while parse_expr parses any expression (1+1 or event or even a subquery (select 1)). It's not possible to parse parts of a select query, such as limit 10.
Placeholders like {where} are just nodes of type ast.Placeholder(field='where'). You can leave them in, and call stmt = replace_placeholders(stmt, { where: parse_expr('1') }) later.
We wrote one AST node ourselves: ast.Constant(value=num_last_days). We did it to santise the value by make sure it's treated as a constant. We might simplify constants further (e.g. parse_const or just {days: 2}), but we're not there yet.

AST nodes

If you want more control, you can build the AST nodes directly. The same query above can be written as:

from posthog.hogql import ast
from posthog.hogql.query import execute_hogql_query
from posthog.hogql.parser import parse_expr

num_last_days = 2

stmt = ast.SelectQuery(
    select=[ast.Field(chain="event"), ast.Field(chain="timestamp")],
    select_from=ast.JoinExpr(table=ast.Field(chain=["events"])),
    where=parse_expr(
        "timestamp > interval {days} day", 
        { 'days': ast.Constant(value=num_last_days) }
    ),
    limit=ast.Constant(value=100),
)

query_result = execute_hogql_query(query=stmt, team=team, query_type="used in logs")
query_result.results == [...]
query_result.columns == ['event', 'timestamp']  # might be useful if you select '*'

You can mix and match parse_expr and ast nodes as you please. The example above still took a shortcut for the where clause because it was easier to write.

Database schema and features

The HogQL database schema is in flux. You will soon be able to explore it in the PostHog app itself.

The most up to date resource is hogql/database.py on Github. At the time of writing, these tables were available:

Python

class Database(BaseModel):
    # Users can query from the tables below
    events: EventsTable = EventsTable()
    persons: PersonsTable = PersonsTable()
    person_distinct_ids: PersonDistinctIdTable = PersonDistinctIdTable()
    session_recording_events: SessionRecordingEvents = SessionRecordingEvents()
    cohort_people: CohortPeople = CohortPeople()
    static_cohort_people: StaticCohortPeople = StaticCohortPeople()

Some tables have some fields that are actually "lazy tables". When accessed they will add a join to the table. The events table is such an example:

Python

class EventsTable(Table):
    uuid: StringDatabaseField = StringDatabaseField(name="uuid")
    event: StringDatabaseField = StringDatabaseField(name="event")
    properties: StringJSONDatabaseField = StringJSONDatabaseField(name="properties")
    timestamp: DateTimeDatabaseField = DateTimeDatabaseField(name="timestamp")
    team_id: IntegerDatabaseField = IntegerDatabaseField(name="team_id")
    distinct_id: StringDatabaseField = StringDatabaseField(name="distinct_id")
    elements_chain: StringDatabaseField = StringDatabaseField(name="elements_chain")
    created_at: DateTimeDatabaseField = DateTimeDatabaseField(name="created_at")

    # lazy table that adds a join to the persons table
    pdi: LazyTable = LazyTable(
        from_field="distinct_id", table=PersonDistinctIdTable(), join_function=join_with_max_person_distinct_id_table
    )
    # person fields on the event itself
    poe: EventsPersonSubTable = EventsPersonSubTable()

    # These are swapped out if the user has PoE enabled
    person: FieldTraverser = FieldTraverser(chain=["pdi", "person"])
    person_id: FieldTraverser = FieldTraverser(chain=["pdi", "person_id"])

If you access pdi.person.properties.$browser, we make a join via person_distinct_ids and persons (these are HogQL table names, not ClickHouse names). We do a bunch of argmax magic in the joins, and inline all accessed properties within the subquery for performance. For the user, it looks just like simple property access.

If you access poe.properties.$browser, we will actually access the field person_properties on the events table.

In practice, you should avoid both and access person.properties.$browser, which will choose the right approach for you.

Add new tables and fields as needed! Just make sure each table has a team_id column.

Query templates

AST nodes

Database schema and features

Questions?

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