Postgresql – Difference between idx_tup_read and idx_tup_fetch on Postgres

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On Postgres 8.4 when you do:

select * from pg_stat_all_indexes where relname = 'table_name';

It returns the fields idx_tup_read and idx_tup_fetch, what is the difference?

Best Answer

When looking at the source code of the view, then you'll see that idx_tup_read is the result of calling pg_stat_get_tuples_returned() and idx_tup_fetch is the result of calling pg_stat_get_tuples_fetched()

The manual describes the two functions as follows:

pg_stat_get_tuples_returned(oid)

Number of rows read by sequential scans when argument is a table, or number of index entries returned when argument is an index

pg_stat_get_tuples_fetched(oid)

Number of table rows fetched by bitmap scans when argument is a table, or table rows fetched by simple index scans using the index when argument is an index

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