I was trying to select all records from the table 'customer'.
I thought I can use the CTRL+ENTER key to execute the query, which effectly equals the B icon in the picture(I thought so).
I quickly concluded that B icon wasn't relevant, then I assume the A icon could execute the query. I clicked it but no result was returned(I hadn't hover enough time over the icon so I fail to notice the icon means 'query tool')
Somehow, I tried to be patient and hover very long enough over each possible icon one by one, and I found that the B icon is query execution button, and it's shortcut is F5
Another stupid error, stupid day.