sql: add some missing postgres vars for ORM compatibility

[jordanlewis]

This PR adds no-op support for setting the following variables, to improve ORM compatability:

• SEARCH_PATH
• CLIENT_ENCODING
• STANDARD_CONFORMING_STRINGS
• CLIENT_MIN_MESSAGES

It also adds support forSHOW SERVER_VERSION, always returning the 9.5.0 postgres version that we return over pgwire.

Resolves #12137
Resolves #12134
Resolves #12113
Resolves #12109
Resolves #12112

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This change is 

[cuongdo]

Review status: 0 of 5 files reviewed at latest revision, 3 unresolved discussions, all commit checks successful.

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pkg/sql/set.go, line 126 at r1 (raw file):

	// See https://www.postgresql.org/docs/9.6/static/runtime-config-logging.html#GUC-APPLICATION-NAME
	case `CLIENT_ENCODING`:
	// See https://www.postgresql.org/docs/9.6/static/multibyte.html

should we return an error if anything but UTF-8 is specified?

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pkg/sql/set.go, line 134 at r1 (raw file):

	// See https://www.postgresql.org/docs/9.6/static/runtime-config-client.html
	case `STANDARD_CONFORMING_STRINGS`:
	// If true, escape backslash literals in strings. We do this by default.

should we return an error or log a warning if an attempt is made to set to disable this? what happens if a client sets this to off and assumes that we honor that?

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pkg/sql/set.go, line 135 at r1 (raw file):

	case `STANDARD_CONFORMING_STRINGS`:
	// If true, escape backslash literals in strings. We do this by default.
	// See https://www.postgresql.org/docs/9.6/static/runtime-config-client.html

This is the right page:
https://www.postgresql.org/docs/9.1/static/runtime-config-compatible.html#GUC-STANDARD-CONFORMING-STRINGS

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Comments from Reviewable

[jordanlewis]

TFTR!

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Review status: 0 of 5 files reviewed at latest revision, 3 unresolved discussions, all commit checks successful.

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pkg/sql/set.go, line 126 at r1 (raw file):

Previously, cuongdo (Cuong Do) wrote…

should we return an error if anything but UTF-8 is specified?

I suppose that would be nice of us, yes. Done.

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pkg/sql/set.go, line 134 at r1 (raw file):

Previously, cuongdo (Cuong Do) wrote…

should we return an error or log a warning if an attempt is made to set to disable this? what happens if a client sets this to off and assumes that we honor that?

Done.

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pkg/sql/set.go, line 135 at r1 (raw file):

Previously, cuongdo (Cuong Do) wrote…

This is the right page:
https://www.postgresql.org/docs/9.1/static/runtime-config-compatible.html#GUC-STANDARD-CONFORMING-STRINGS

Done.

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Comments from Reviewable

[cuongdo]

Review status: 0 of 5 files reviewed at latest revision, 3 unresolved discussions, some commit checks pending.

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pkg/sql/set.go, line 126 at r1 (raw file):

Previously, jordanlewis (Jordan Lewis) wrote…

I suppose that would be nice of us, yes. Done.

Can you also add a test for the error case?

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pkg/sql/set.go, line 134 at r1 (raw file):

Previously, jordanlewis (Jordan Lewis) wrote…

Done.

Could you add a test for the error case?

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pkg/sql/set.go, line 151 at r3 (raw file):

		switch parser.Name(s).Normalize() {
		case parser.ReNormalizeName("off"):
			return nil, fmt.Errorf("STANDARD_CONFORMING_STRINGS=off not supported")

nit: errors.New instead of fmt.Errorf?

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Comments from Reviewable

[jordanlewis]

Review status: 0 of 5 files reviewed at latest revision, 3 unresolved discussions.

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pkg/sql/set.go, line 126 at r1 (raw file):

Previously, cuongdo (Cuong Do) wrote…

Can you also add a test for the error case?

Done.

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pkg/sql/set.go, line 134 at r1 (raw file):

Previously, cuongdo (Cuong Do) wrote…

Could you add a test for the error case?

Done.

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pkg/sql/set.go, line 151 at r3 (raw file):

Previously, cuongdo (Cuong Do) wrote…

nit: errors.New instead of fmt.Errorf?

Simplified this case.

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Comments from Reviewable

[cuongdo]

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Review status: 0 of 5 files reviewed at latest revision, 3 unresolved discussions, all commit checks successful.

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Comments from Reviewable

[jseldess]

In the future, when we have a docs page collecting session variables, these might be included.