sqlmesh re-runs backfills for lookback intervals when you re-apply the same plan

[sudokai]

When I re-apply the same plan, a model with a custom cron expression (not @daily) should not re-run backfills for lookback intervals, but it does so.

I can reproduce it with this setup (BigQuery, sqlmesh 0.115.1):

model (
    name staging.test5
    , kind incremental_by_time_range (
        time_column (event_timestamp, '%Y-%m-%d %H:%M:%S.%f')
        , lookback 2
    )
    , partitioned_by timestamp_trunc(event_timestamp, day)
    , cron '0 12 * * *'
    , start '2024-08-01'
);

select
    event_timestamp::timestamp
from staging.seed
where
    event_timestamp >= @start_ts
    and event_timestamp < @end_ts
;

model (
    name staging.seed
    , kind seed (
        path '$root/seeds/events.csv'
    )
);

CSV test data:

event_timestamp,event_name,user_id
2024-08-06 00:00:00.000000 UTC,login,1234
2024-08-02 00:00:00.000000 UTC,login,1234
2024-08-04 00:00:00.000000 UTC,login,1236
2024-08-03 00:00:00.000000 UTC,login,1235
2024-08-05 00:00:00.000000 UTC,login,1237

The command sequence and outputs:

user@mac ~/D/sqlmesh-example [1]> sqlmesh plan -v                                   
New environment `prod` will be created from `prod`
Summary of differences against `prod`:
Models:
└── Added:
    ├── staging.seed
    └── staging.test5
Models needing backfill (missing dates):
├── staging.seed: 2024-08-05 - 2024-08-05
└── staging.test5: 2024-08-01 - 2024-08-04
Apply - Backfill Tables [y/n]: y
staging.seed created
staging.test5 created
Creating physical table ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 100.0% • 2/2 • 0:00:16

All model versions have been created successfully

[1/1] staging.seed evaluated in 9.57s
[1/1] staging.test5 evaluated in 17.24s
Evaluating models ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 100.0% • 2/2 • 0:00:26                                                         
                                                                                                                                          

All model batches have been executed successfully

staging.seed promoted
staging.test5 promoted
Virtually Updating 'prod' ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 100.0% • 0:00:02

The target environment has been updated successfully

user@mac ~/D/sqlmesh-example> sqlmesh plan -v                                        
No differences when compared to `prod`
Models needing backfill (missing dates):
└── staging.test5: 2024-08-03 - 2024-08-04
Apply - Backfill Tables [y/n]: y
staging.test5 created
staging.seed created
Creating physical table ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 100.0% • 2/2 • 0:00:02

All model versions have been created successfully

[1/1] staging.test5 evaluated in 11.82s
Evaluating models ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 100.0% • 1/1 • 0:00:11                                                         
                                                                                                                                          

All model batches have been executed successfully

Virtually Updating 'prod' ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 0.0% • 0:00:14

The target environment has been updated successfully

user@mac ~/D/sqlmesh-example> sqlmesh plan -v                              
No differences when compared to `prod`
Models needing backfill (missing dates):
└── staging.test5: 2024-08-03 - 2024-08-04
Apply - Backfill Tables [y/n]: