release-21.2: sql: avoid string formatting in reportSessionDataChanges when not necessary

[nvanbenschoten]

Backport 1/1 commits from #74338.

/cc @cockroachdb/release

Release justification: avoids superfluous heap allocations on each transaction.

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This commit updates each of the bufferableParamStatusUpdates
sessionVars to have an Equal function, which returns whether the
value of the variable is equal between two SessionData references.
This allows connExecutor.reportSessionDataChanges to compare the
values of each of the variables without resorting to string formatting.
Instead, the variables are only string formatted if they have actually
changed and need to be reported to the client.

Micro-benchmarks have revealed that the string formatting for variables
like "DateStyle" is a meaningful expense to incur on each transaction,
especially for small OLTP transactions.

name                   old time/op    new time/op    delta
KV/Scan/SQL/rows=1-10    94.7µs ± 8%    92.9µs ± 2%    ~     (p=0.762 n=10+8)

name                   old alloc/op   new alloc/op   delta
KV/Scan/SQL/rows=1-10    20.1kB ± 0%    19.9kB ± 0%  -1.00%  (p=0.000 n=10+10)

name                   old allocs/op  new allocs/op  delta
KV/Scan/SQL/rows=1-10       245 ± 0%       235 ± 0%  -4.08%  (p=0.000 n=10+9)

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Patches should only be created for serious issues or test-only changes.
• Patches should not break backwards-compatibility.
• Patches should change as little code as possible.
• Patches should not change on-disk formats or node communication protocols.
• Patches should not add new functionality.
• Patches must not add, edit, or otherwise modify cluster versions; or add version gates.

If some of the basic criteria cannot be satisfied, ensure that the exceptional criteria are satisfied within.

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters.
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.

Add a brief release justification to the body of your PR to justify this backport.

Some other things to consider:

• What did we do to ensure that a user that doesn’t know & care about this backport, has no idea that it happened?
• Will this work in a cluster of mixed patch versions? Did we test that?
• If a user upgrades a patch version, uses this feature, and then downgrades, what happens?

[cockroach-teamcity]

This change is