Github user twalthr commented on the issue:
https://github.com/apache/flink/pull/3107
@wuchong if `toString` is called for the first time. It calls
`tEnv.registerTableInternal(this)` which uses an atomic counter and generates a
unique identifier such as `UnnamedTable$232`. This identifier is returned and
saved in the table. Every further call of toString return this identifier.
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