[haacked]

Problem

Cohort filters using IS_SET and IS_NOT_SET operators have inconsistent behavior compared to other PostHog systems when handling string "null" values. Users with properties containing the literal string "null" are incorrectly included in IS_SET filters and excluded from IS_NOT_SET filters, which contradicts how feature flags and other parts of PostHog handle these values.

Fixes #29916

Changes

• Modified /posthog/hogql/property.py: Updated _expr_to_compare_op function to handle string "null" values consistently
• IS_SET logic: Now excludes both SQL NULL and string "null" values: (property != NULL) AND (property != 'null')
• IS_NOT_SET logic: Now includes both SQL NULL and string "null" values: (property = NULL) OR (property = 'null') OR (NOT JSONHas(...))

Behavior Truth Table

Property Value	Type	IS_SET (Before)	IS_SET (After)	IS_NOT_SET (Before)	IS_NOT_SET (After)
NULL (SQL NULL)	SQL NULL	❌ FALSE	✅ FALSE	✅ TRUE	✅ TRUE
Missing key	Missing	❌ FALSE	✅ FALSE	✅ TRUE	✅ TRUE
"null" (string)	String	❌ TRUE (BUG)	✅ FALSE (FIXED)	❌ FALSE (BUG)	✅ TRUE (FIXED)
"" (empty string)	String	✅ TRUE	✅ TRUE	✅ FALSE	✅ FALSE
"value"	String	✅ TRUE	✅ TRUE	✅ FALSE	✅ FALSE
0	Number	✅ TRUE	✅ TRUE	✅ FALSE	✅ FALSE
false	Boolean	✅ TRUE	✅ TRUE	✅ FALSE	✅ FALSE

Rationale for String "null" Handling

I had concerns about treating "null" as null because it could be a valid value.

However, after I looked around other code, it seems that **PostHog already treats string "null" as "not set" in other areas. For example,

Materialized Columns (existing behavior):

# From posthog/hogql/printer.py
# LEGACY_NULL_AS_NULL mode explicitly converts string "null" to SQL NULL
materialized_property_sql = f"nullIf(nullIf({column}, ''), 'null')"

I think this has to do with the fact that string "null" values can arise from:

• Data ingestion issues where null values are serialized as strings
• Third-party systems that represent null as string literals
• Import processes that convert database nulls to string representations

How did you test this code?

Automated Testing

• Unit Tests
• Manual tests

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