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of known-equal expressions includes any constant expressions (including Params from outer queries), we actively suppress any 'var = var' clauses that are or could be deduced from the set, generating only the deducible 'var = const' clauses instead. The idea here is to push down the restrictions implied by the equality set to base relations whenever possible. Once we have applied the 'var = const' clauses, the 'var = var' clauses are redundant, and should be suppressed both to save work at execution and to avoid double-counting restrictivity.