plan: compute all inner joins in memory if they fit

[erizocosmico]

Fixes #577

Because we do not have a way to estimate the cost of each side of
a join, it is really difficult to know when we can compute one in
memory. But not doing so, causes inner joins to be painfully slow,
as one of the branches is iterated multiple times.

This PR addresses this by ensuring that if the right branch of the
inner join fits in memory, it will be computed in memory even if
the in-memory mode has not been activated by the user.

An user can set the maximum threshold of memory the gitbase server
can use before considering the joins should not be performed in
memory using the MAX_MEMORY_INNER_JOIN environment variable or
the max_memory_joins session variable specifying the number of
bytes. The default value for this is the half of the available
physical memory on the operating system.

Because previously we had two iterators: innerJoinIter and
innerJoinMemoryIter, and now innerJoinIter must be able to do
the join in memory, innerJoinMemoryIter has been removed and
innerJoinIter replaced with a version that can work with three
modes:

• unknownMode we don't know yet how to perform the join, so keep
  iterating until we can find out. By the end of the first full pass
  over the right branch unknownMode will either switch to
  multipassMode or memoryMode.
• memoryMode which computes the rest of the join in memory. The
  iterator can have this mode before starting iterating if the user
  activated the in memory join via session or environment vars, in
  which case it will load all the right side on memory before doing
  any further iteration. Instead, if the iterator started in
  unknownMode and switched to this mode, it's guaranteed to already
  have loaded all the right side. From that point on, they work in
  exactly the same way.
• multipassMode, which was the previous default mode. Iterate the
  right side of the join for each row in the left side. More expensive,
  but less memory consuming. The iterator can not start in this mode,
  and can only be switched to it from unknownMode in case the
  memory used by the gitbase server exceeds the maximum amount of memory
  either set by the user or by default.

Signed-off-by: Miguel Molina miguel@erizocosmi.co

[ajnavarro]

LGTM after having a look to requested changes. Would be great to have this documented on gitbase. Should we open an issue on gitbase to do not forget to update https://docs.sourced.tech/gitbase/using-gitbase/optimize-queries ?

[erizocosmico]

Fixed

[erizocosmico]

Shall we merge this? @ajnavarro