Thread: counting disk access from index seek operation -- how to?

I need a way to tell how many pages loaded from disk for a particular
index seek operation.

What I did is to set a global flag to true before calling the
following statement
(inside index_getnext() in "/backend/access/indexam.c")
 found = DatumGetBool(FunctionCall2(&scan->fn_getnext,
PointerGetDatum(scan),                                                     
Int32GetDatum(direction)));


then for each access to disk, I increment a counter until the above call is
finished and set back the global flag.

The number of page IOs is not even matching from what I got from
"select * from pg_stat_all_indexes"  ---  (is there anything I need to
set in postgresql.conf?)

also, the number of paged IOs for a given index seek is always less
than the total
page loads. Is it because of the statement (following the above in
"/backend/access/indexam.c")?
"if (heap_release_fetch(scan->heapRelation, scan->xs_snapshot,                             heapTuple, &scan->xs_cbuf,
true,  &scan->xs_pgstat_info))" 

If I am running in a single user mode, is there a way to avoid using
extra page IO in
the above statement? It seems to me the extra page IO is caused by
comparing snapshots...

thanks

"huaxin zhang" <uwcssa@gmail.com> wrote
>I need a way to tell how many pages loaded from disk for a particular
> index seek operation.

By pages loaded, you mean physically or logically? In either ways, I would 
suggest you to take a look at _bt_getbuf().

>
> What I did is to set a global flag to true before calling the
> following statement
> (inside index_getnext() in "/backend/access/indexam.c")
>

This is not the right place. index_getnext() returns when it find a 
satisifed tuple or no match at all. Thus it may access many pages, from the 
root of btree down to some leaf node.

> also, the number of paged IOs for a given index seek is always less
> than the total
> page loads. Is it because of the statement (following the above in
> "/backend/access/indexam.c")?
>
> "if (heap_release_fetch(scan->heapRelation, scan->xs_snapshot,
>  heapTuple, &scan->xs_cbuf, true,
>    &scan->xs_pgstat_info))"
>

This functions checks if the real data on the heap matches the information 
indicated by the index, since we just save a key and pointer to the real 
data on index. Thus, extra IOs may needed.

Regards,
Qingqing