I have a table that has one boolean column.
productid integer
isactive boolean
When I execute the query
SELECT productid   
    FROM 
    product  
    WHERE ispublish
    LIMIT 15 OFFSET  0

After that, I created an index for the ispublish column:
CREATE INDEX idx_product_ispublish ON product USING btree (ispublish)
and re-execute
SELECT productid   
       FROM 
       product  
       WHERE ispublish
       LIMIT 15 OFFSET  0
The result:

=> No difference
I've been tried the following, but the results are the same:
CREATE INDEX idx_product_ispublish ON product USING btree (ispublish)
CREATE INDEX idx_product_ispublish ON product USING btree (ispublish)
CREATE INDEX idx_product_ispublish ON product (ispublish) WHERE ispublish is TRUE
Who can explain that to me?
 
    