in a nutshell

read by other session
- 'n' people wanted a block.
- the block wasn't in memory so we need to read it off disk
- it doesnt make sense for 'n' people to read the disk
- so 1 will read it into memory, the others will wait

buffer busy waits
- i've got a block in memory
- 'n' people want to access/modify that block in way that may be incompatible with others
- someone has to wait

enq: SQ - contention
- lots of people accessing a sequence
- at some point, several of them want to update the dictionary table that holds the last number cached

db file sequential read
- i want to read a single block from disk.
- that takes time

enq: HW - contention
- a table is growing
- 'n' people are all adding data to it
- at some point we need to shift the high watermark for the table
- people need to wait whilst one of them does that.