Data on UI

Tell us more about the problem. The objects involved, the data size, screenshots, code, and anything that adds more light to the issue. 

We are quering data from DB and showing on the UI that table has around 5L records. 

I want to understand in which scenarios the data would not load. How can we classify at an higher level.

1. You don't have access to the DataStore
2. You don't have access to the recordType
3. You are not in the record level security configuration
4. You have some default filters working 
5. The record type is not synced
6. The connection to DB is broken
7. The mapping between your CDT and table in the DB in invalid 

What could be the case when we don't have data on Appian 1 AM and it is available at 2 AM, both the times the data is available in the DB.

An overnight sync happens somewhere between 1 and 2 AM. Assuming you are using records. 

yeah, got it.

One last scenario, 

What could be the case

when the data is available on 1st Jan 10 PM and

we don't have data on UI on 2nd Jan Appian 1 AM and

it is available at 2nd Jan 5 PM, all the times the data is available in the DB.

Is your DB connected with some other application, outside Appian, that changes the data?

No

And tell me if you have any API that writes data in the table. Any source from which the data is getting written to the DB other than "Write Records"?

Nothing

Harshit Bumb (Appyzie) said:

• You don't have access to the DataStore
• You don't have access to the recordType
• You are not in the record level security configuration
• You have some default filters working 
• The record type is not synced
• The connection to DB is broken
• The mapping between your CDT and table in the DB in invalid 

Can't think of anything other than these. If it happened only once, there could be one of these problems. 
You can try to check for the logs of that day if they are still available. And if this happens regularly, then I think the right time to debug is when the data isn't coming up. That time, you can test objects individually to see what is causing this issue. 

Harshit Bumb (Appyzie) said:

• You don't have access to the DataStore
• You don't have access to the recordType
• You are not in the record level security configuration
• You have some default filters working 
• The record type is not synced
• The connection to DB is broken
• The mapping between your CDT and table in the DB in invalid 

Can't think of anything other than these. If it happened only once, there could be one of these problems. 
You can try to check for the logs of that day if they are still available. And if this happens regularly, then I think the right time to debug is when the data isn't coming up. That time, you can test objects individually to see what is causing this issue. 

Sure, Thanks

Can you please mention more details about the logs. What kind of logs are available?

You can look for sync history in the record type to see if it failed someday.