plpgsql: move barrier after outer-join for SQL stmt with INTO clause

[DrewKimball]

PL/pgSQL routines use barrier expressions to prevent optimizations that would remove side effecting expressions from the query plan. Previously, we would add a barrier below the outer join used to ensure at least one row returned by a SQL statement with an INTO clause. However, this wasn't enough, since after some inlining and projection push-down into the join, the join could be eliminated. This could result in a side-effecting SQL statement being dropped if its target variable(s) weren't referenced.

This commit fixes the bug by moving the barrier above the outer-join. This prevents optimizations (like join elimination) that rely on proving that the columns from the null-extended side of the join are not needed.

Fixes #147269

Release note (bug fix): Fixed a bug existing since SQL statements with INTO clause were introduced for PL/pgSQL routines in v23.2. The bug could cause a SQL statement with side effects (e.g. INSERT) to be dropped if none of the target variables from the INTO clause were referenced.

[cockroach-teamcity]

This change is 

[yuzefovich]

Nice find! Worth backporting perhaps all the way to 24.1?

@yuzefovich reviewed 4 of 4 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @mw5h)

[mgartner]

Great find!

@mgartner reviewed 4 of 4 files at r1, all commit messages.
Reviewable status: complete! 2 of 0 LGTMs obtained (waiting on @mw5h)

[michae2]

Piling on to say nice job!

@michae2 reviewed 4 of 4 files at r1, all commit messages.
Reviewable status: complete! 3 of 0 LGTMs obtained (waiting on @mw5h)

[mw5h]

@mw5h reviewed all commit messages.
Reviewable status: complete! 3 of 0 LGTMs obtained (waiting on @DrewKimball)

[DrewKimball]

TFTRs!

bors r+

[craig]

Build failed:

• unit_tests

[DrewKimball]

Looks like a flake.

bors retry

[craig]

Build succeeded:

• examples_orms
• docker_image_amd64
• unit_tests
• acceptance
• linux_amd64_build
• check_generated_code
• local_roachtest
• linux_amd64_fips_build
• lint
• local_roachtest_fips

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💡 Consider backporting to the fork repo instead of the main repo. See instructions for more details.

💡 Consider backporting to the fork repo instead of the main repo. See instructions for more details.

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