Currently, although using both dirty tracking (ivar backed and
attributes backed) on one model is not supported (doesn't fully work at
least), both dirty tracking are being performed, that is very slow.
As long as attributes backed dirty tracking is used, ivar backed dirty
tracking should not need to be performed.
I've refactored to extract new `ForcedMutationTracker` which only tracks
`force_change` to be performed for ivar backed dirty tracking, that
makes dirty tracking on Active Record 2x ~ 30x faster.
https://gist.github.com/kamipo/971dfe0891f0fe1ec7db8ab31f016435
Before:
```
Warming up --------------------------------------
changed? 4.467k i/100ms
changed 5.134k i/100ms
changes 3.023k i/100ms
changed_attributes 4.358k i/100ms
title_change 3.185k i/100ms
title_was 3.381k i/100ms
Calculating -------------------------------------
changed? 42.197k (±28.5%) i/s - 187.614k in 5.050446s
changed 50.481k (±16.0%) i/s - 246.432k in 5.045759s
changes 30.799k (± 7.2%) i/s - 154.173k in 5.030765s
changed_attributes 51.530k (±14.2%) i/s - 252.764k in 5.041106s
title_change 44.667k (± 9.0%) i/s - 222.950k in 5.040646s
title_was 44.635k (±16.6%) i/s - 216.384k in 5.051098s
```
After:
```
Warming up --------------------------------------
changed? 24.130k i/100ms
changed 13.503k i/100ms
changes 6.511k i/100ms
changed_attributes 9.226k i/100ms
title_change 48.221k i/100ms
title_was 96.060k i/100ms
Calculating -------------------------------------
changed? 245.478k (±16.1%) i/s - 1.182M in 5.015837s
changed 157.641k (± 4.9%) i/s - 796.677k in 5.066734s
changes 70.633k (± 5.7%) i/s - 358.105k in 5.086553s
changed_attributes 95.155k (±13.6%) i/s - 470.526k in 5.082841s
title_change 566.481k (± 3.5%) i/s - 2.845M in 5.028852s
title_was 1.487M (± 3.9%) i/s - 7.493M in 5.046774s
```
When an array of hashes is added to a `HashWithIndifferentAccess`, the
hashes are replaced with HWIAs by mutating the array in place.
If an attribute's value is an array of hashes, `changes_to_save` will
convert it to an array of HWIAs as a side-effect of adding it to the
changes hash.
Using `merge!` instead of `[]=` fixes the problem, as `merge!` copies
any array values in the provided hash instead of mutating them.
`changed_attribute_names_to_save` is called in `keys_for_partial_write`,
which is called on every save when partial writes are enabled.
We can avoid generating the full changes hash by asking the mutation
tracker for just the names of the changed attributes. At minimum this
saves one array allocation per attribute, but will also avoid calling
`Attribute#original_value` which is expensive for serialized attributes.
#30985 caused `object.save` performance regression since calling
`changes` in `changes_applied` is very slow.
We don't need to call the expensive method in `changes_applied` as long
as `@attributes` is tracked by mutation tracker.
https://gist.github.com/kamipo/1a9f4f3891803b914fc72ede98268aa2
Before:
```
Warming up --------------------------------------
create_string_columns
73.000 i/100ms
Calculating -------------------------------------
create_string_columns
722.256 (± 5.8%) i/s - 3.650k in 5.073031s
```
After:
```
Warming up --------------------------------------
create_string_columns
96.000 i/100ms
Calculating -------------------------------------
create_string_columns
950.224 (± 7.7%) i/s - 4.800k in 5.084837s
```
