[YSQL] Select on column used in partial index does not use the index

[ndeodhar]

After creating a partial index on a column, if we run a select query with a condition based on that column, we do a full scan of the table instead of using the index:

CREATE INDEX discontinued_products
ON products(discontinued)
WHERE discontinued = 1;

EXPLAIN SELECT
   product_id,
   product_name,
   discontinued
FROM
   products
WHERE
   discontinued = 1;


Foreign Scan on products  (cost=0.00..102.50 rows=1000 width=104)

[ndeodhar]

Fixed as a result of #4494.