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After executing the query provided here, I got below result 

Select  pg_relation_size(20473, 'main'),pg_relation_size(20473,'fsm'),
pg_relation_size(20473,'vm'),pg_relation_size(20473,'init'),pg_table_size(20473)
from pg_statio_user_tables

enter image description here.

Why am I not getting 

pg_relation_size(20473,'main') + pg_relation_size(20473,'fsm') + pg_relation_size(20473,'vm') + pg_relation_size(20473,'init') = pg_table_size(20473)?
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YogeshR
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2 Answers2

2

pg_relation_size calls calclulate_relation_size.

pg_table_size calls calculate_table_size.

calculate_table_size calls calclulate_relation_size and calculate_toast_table_size.

So when calling pg_table_size the TOAST-Data is added.

Link to Source Code

Ben H
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2

From the documentation:

pg_relation_size(relation regclass, fork text)
Disk space used by the specified fork ('main', 'fsm', 'vm', or 'init') of the specified table or index

pg_table_size(regclass)
Disk space used by the specified table, excluding indexes (but including TOAST, free space map, and visibility map)

So it must be the TOAST table.

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Laurenz Albe
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