code: func SetPstmtIDSchemaVersion(key kvcache.Key, pstmtID uint32, schemaVersion int64) {
  	psStmtKey, isPsStmtKey := key.(*pstmtPlanCacheKey)
  	if !isPsStmtKey {
  		return
  	}
  	psStmtKey.pstmtID = pstmtID
  	psStmtKey.schemaVersion = schemaVersion
  	psStmtKey.hash = psStmtKey.hash[:0]
  }
docs: // SetPstmtIDSchemaVersion implements PstmtCacheKeyMutator interface to change pstmtID and schemaVersion of cacheKey.
  // so we can reuse Key instead of new every time.