Hi,

While reviewing [1], I noticed several cases where BlockNumber seems to be 
misused.

Although BlockNumber is currently underlying defined as uint32, it has a 
special meaning. For example:
```
#define InvalidBlockNumber ((BlockNumber) 0xFFFFFFFF)
#define MaxBlockNumber ((BlockNumber) 0xFFFFFFFE)
```
So my understanding is that BlockNumber should only be used to identify a block.

However, I saw several places where variables of type BlockNumber are actually 
used as counts. For example:
```
typedef struct LVRelState
{

    BlockNumber blkno;   <== correct usage

    BlockNumber rel_pages; /* total number of pages */  <== mis-use
```
In this example, rel_pages is actually a count. In theory it could just be an 
int (or some other count type).

Another example:
```
static void
system_time_samplescangetsamplesize(PlannerInfo *root,
                                                                        
RelOptInfo *baserel,
                                                                        List 
*paramexprs,
                                                                        
BlockNumber *pages,
                                                                        double 
*tuples)
```
Here the parameter pages is also a count indicating the number of pages.

So I want to confirm whether my understanding is correct that these are misuses 
of BlockNumber. If so, would it be worth a cleanup patch to fix such cases?

[1] https://postgr.es/m/[email protected]

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/






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