Teach forder to re-use existing key and index attributes instead of sorting from scratch

[jangorecki]

Closes #4380
Closes #4387
Partially address #2947 for index, not for keys. I don't think we can do much better there so I would also close that one.
Towards #2879
Resolves #1321 (a test should be added)

---

Examples below.
Note that output below is missing newly added attributes populated by new option retStats=TRUE. I am not adding them below to make output easier to read. retStats=TRUE will just return more attributes (all comes for free, unlike retGrp=T): any-not-ascii, any-not-utf8, any-na, any-inf-nan.

## either of
# cc()
# library(data.table); forderv = data.table:::forderv

• setindex[v] will now compute groups (retGrp=T) as well, and store it together with index attribute. Note that it only applies to setindex interface, any internal index computation will use forder and will not compute groups unnecessarily (explained in computing index could find groups as well #4387).

d2 = data.table(id=2:1, v2=1:2)
setindexv(d2, "id")
str(attr(attr(d2, "index"), "__id"))
# int [1:2] 2 1
# - attr(*, "starts")= int [1:2] 1 2
# - attr(*, "maxgrpn")= int 1
d2 = data.table(id=2:1, v2=1:2)
d2[id==1L]
attr(attr(d2, "index"), "__id")
# int [1:2] 2 1

• Low level C short circuit to re-use existing keys in forder (allow re-use of index during C bmerge which was calling C forder directly merge could utilize i index #4380). Verbose codes: key opt=1, indices opt=2.

d1 = data.table(id=1:2, v1=1:2)
d2 = data.table(id=2:1, v2=1:2)
setindexv(d2, "id")
d1[d2, on="id", verbose=TRUE]
#forderLazy: using existing index: __id
#forderLazy: opt=2, took 0.000s

• Newly computed index could be now set as DT index attribute directly from C, next to where it is being computed. It means that forderv could already set the index. It is disabled by default, can be enabled by option datatable.forder.auto.index=TRUE. This option is only for debugging/development, not going to be exported. Ultimately current datatable.auto.index option will control that.

options(datatable.verbose=TRUE, datatable.forder.auto.index=TRUE)
d2 = data.table(id=2:1, v2=1:2)
o = forderv(d2, "id")
#forderLazy: setting index (retGrp=0) on DT: __id
#forderLazy: opt=-1, took 0.000s
str(o)
# int [1:2] 2 1
o = forderv(d2, "id", retGrp=TRUE) # has to compute groups
#forderLazy: index found but no retGrp: __id
#forderLazy: setting index (retGrp=1) on DT: __id
#forderLazy: opt=-1, took 0.000s
str(o)
# int [1:2] 2 1
# - attr(*, "starts")= int [1:2] 1 2
# - attr(*, "maxgrpn")= int 1
o = forderv(d2, "id", retGrp=FALSE) # don't have to compute
#forderLazy: using existing index: __id
#forderLazy: opt=2, took 0.000s
str(o)
# int [1:2] 2 1
o = forderv(d2, "id", retGrp=TRUE) # groups info still there
#forderLazy: using existing index: __id
#forderLazy: opt=2, took 0.000s
str(o)
# int [1:2] 2 1
# - attr(*, "starts")= int [1:2] 1 2
# - attr(*, "maxgrpn")= int 1

## other functions which uses forder can benefit as well
DT = data.table(A = rep(1:3, each=4), B = rep(1:4, each=3), C = rep(1:2, 6))
uniqueN(DT)
#forderLazy: setting index (retGrp=1) on DT: __A__B__C
#forderLazy: opt=-1, took 0.000s
#[1] 10
str(attr(attr(DT,"index"),"__A__B__C"))
# int [1:12] 1 3 2 4 5 6 7 8 9 11 ...
# - attr(*, "starts")= int [1:10] 1 3 4 5 6 7 8 9 10 11
# - attr(*, "maxgrpn")= int 2
uniqueN(DT)
#forderLazy: using existing index: __A__B__C
#forderLazy: opt=2, took 0.000s
#[1] 10

It is disabled by default because of the issue caused by #507. forderv writes an attribute by reference, when invoked for example via uniqueN by group, it writes the index attribute to the first group .SD, the same index attribute is later re-used for further groups, making results incorrect:

# test 1475.02
options(datatable.verbose=TRUE, datatable.forder.auto.index=TRUE)
DT = data.table(A = rep(1:3, each=4), B = rep(1:4, each=3), C = rep(1:2, 6))
DT[, .(uN=uniqueN(.SD)), by=A]
#forder: setting index (retGrp=1) on DT: __B__C
#forder: opt=-1, took 0.000s
#forder: using existing index: __B__C
#forder: opt=2, took 0.000s
#forder: using existing index: __B__C
#forder: opt=2, took 0.000s

If we will manage to detect that a call has been invoked from j+by group, then we can set lazy=FALSE to escape all optimizations. At the same time we could setDTthreads to 1 in such case #4294.

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@@            Coverage Diff             @@
##           master    #4386      +/-   ##
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+ Coverage   97.47%   97.50%   +0.02%     
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[jangorecki]

there is a related comment next to test 1419.62

# checks also that the prior index a is dropped (because y is keyed with no index)

but we should not drop that index, as long as order does not change then index is still valid, and it can even be useful now, because it can store groups info.