Deprecate @ based syntax for accessing iceberg snapshots

[phd3]

Description

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this deprecates @ based syntax for accessing iceberg snapshots.

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iceberg connector

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# Iceberg
* Deprecate Iceberg snapshot access using mangled table names, in favor of the `AS OF` syntax. The old behavior can be restored by `allow_legacy_snapshot_syntax`  session property or `iceberg.allow-legacy-snapshot-syntax` config property. ({issue}`10768`)

[b-slim]

Thanks, @phd3 for the patch I am wondering why shall we keep around this ambiguous access, I think it is better to fail when a snapshot id is not valid and the user can still access the latest snapshot when using table name with no snapshot. IOW this flag is yet another flag with no clear added value?

[phd3]

I think it is better to fail when a snapshot id is not valid

Current behavior also allows interpreting that number as a TS. Since this syntax has been around for a while, IMO would be worth providing a switch before fully removing. but let's see what others think too

[phd3]

Flaky #10752

[martint]

AS OF syntax is already merged into Trino, BTW.

[electrum]

Should we make this config an enum of BOTH, SNAPSHOT_ONLY, NONE?

[phd3]

@electrum updated to add enums. would you like to take another look?

[findepi]

Once AS OF syntax support is merged, we can use the same conf to get rid of the syntax support altogether.

#10258 is merged now.

[phd3]

@findepi @electrum is allow/deny sufficient now since #10258 merged? or still want to keep enum?

[findepi]

@phd3 i am fine with simple boolean flag

[phd3]

@findepi ptal