The sql can be predigest to:
select random() from a where random()<0.5;
\d a
Table "public.a"
Column | Type | Modifiers
--------+---------+-----------
a | integer |
This query can get about a half rows in the table 'a'.
When search a row, postgresql calcuate random() twice.
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TIP 9: the planner will ignore your desire to choose an index scan if your
joining column's datatypes do not match
