Re: Unable to understand index only scan as it is not happening for one table while it happens for other

Thanks.

Now I did the same query, but it is going for *index-only scan* only after I
put *limit*

localdb=# explain analyse verbose select uid from mm where uid>100 order by
uid;
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------
Sort (cost=30.99..31.87 rows=354 width=8) (actual time=0.243..0.294
rows=354 loops=1)
Output: uid
Sort Key: mm.uid
Sort Method: quicksort Memory: 41kB
-> Seq Scan on public.mm (cost=0.00..16.00 rows=354 width=8) (actual
time=0.010..0.123 rows=354 loops=1)
Output: uid
Filter: (mm.uid > 100)
Rows Removed by Filter: 46
Planning time: 0.109 ms
Execution time: 0.342 ms
(10 rows)

localdb=# explain analyse verbose select uid from mm where uid>100 order by
uid *limit 10*;
QUERY
PLAN
--------------------------------------------------------------------------------------------------------------------------------------------------------
Limit (cost=0.27..2.13 rows=10 width=8) (actual time=0.026..0.037 rows=10
loops=1)
Output: uid
-> Index Only Scan using mm_pkey on public.mm (cost=0.27..65.91
rows=354 width=8) (actual time=0.025..0.034 rows=10 loops=1)
Output: uid
Index Cond: (mm.uid > 100)
Heap Fetches: 10
Planning time: 0.096 ms
Execution time: 0.059 ms
(8 rows)

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Thanks,
Rajan.
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