1: According to my understanding, there is no row open/close, just has column open/close with SDRAM.
2: For column open/close: I understanding is: the SDRAM is so big that you must identify which block you want to do write/read access to improve timing. For example: if the SDRAM is 1Mx16B. Then need decode 20bit address, the delay is huge. If you do column open, you can decode 8bit or 10bit address in advance, the in each read/write access you just need decode 12/10 bit address.
Because you has open a column, when you want to do write/read access on other column, of cause you need close the opned column first then open other column.