New wal format distorts pg_xlogdump --stats

Hi,

The new WAL format calculates FPI vs plain record data like:
rec_len = XLogRecGetDataLen(record) + SizeOfXLogRecord;
fpi_len = record->decoded_record->xl_tot_len - rec_len;

Due to the amount of data now handled outside the main data portion ,
that doesn't seem correct to me. As an example, a couple thousand
inserts with full_page_writes=off now yields:

Type N (%) Record size (%) FPI size (%) Combined size (%)
---- - --- ----------- --- -------- --- ------------- ---
Heap/INSERT 30167 ( 99.53) 814509 ( 99.50) 965856 ( 99.54) 1780365 ( 99.52)

I think fpi_len now needs to only count the sum of the of the actual
length of block images, and all the rest needs to be rec_len.

Greetings,

Andres Freund

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Andres Freund http://www.2ndQuadrant.com/
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