How to reference DBInstanceVCPU in RDS parameter?

Question

Per [the docs here](https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_ParamValuesRef.html#USER_FormulaVariables), `DBInstanceVCPU` should be supported.

However, I've noted that in the `default.aurora-postgresql13` this seems to be formatted differently (note the `$` vs lack thereof):

```
max_connections : LEAST({DBInstanceClassMemory/9531392},5000)
max_parallel_workers : GREATEST(${DBInstanceVCPU/2},8)
```

Moreover, if I try to create a custom parameter group and edit `max_parallel_workers` by simply changing the `8` to another integer, the UI refuses to accept anything with `DBInstanceVCPU` (with or without the `$`).

Is this an issue with the RDS UI's parameter validation or is there something else that I'm missing?

Thanks!

Accepted Answer

Hello,

Please note, I confirm this is a known issue and currently the "DBInstanceVCPU" cannot be used/re-used in parameter value when being modified from customer's end. This makes "DBInstanceVCPU" unusable for customer. Apologies, that AWS public documentation [1] is misleading and I will report it to our internal team.

```
Example:  
max_parallel_workers={DBInstanceVCPU/4*3} ----> is not accepted as a valid entry
```
```
Example:
max_parallel_workers=GREATEST({DBInstanceVCPU*3/4},1) ----> is accepted as a valid entry
```

**Reference**

[1] Working with DB parameter groups - DB parameter formulas - 
https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_WorkingWithParamGroups.html#USER_ParamFormulas

Hope it helps and if it does, I would appreciate if answer can be accepted so that community can benefit for clarity when searching for similar enquiries in repost/aws guide.

Thank you!

How to reference DBInstanceVCPU in RDS parameter?

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