Hi, I'm working on Beam SQL and we are using Apache Calcite for our query parsing and optimization. We are trying to use row count estimates for Volcano Optimizer. Currently, Calcite returns row count estimate of 100 for every table. It seems the cost estimate comes from bunch of handlers in org.apache.calcite.rel.metadata.RelMetadataQuery; however, apparently the handlers are automatically generated and I cannot figure out how I can pass my own handler for cost estimation. Also, I am not even sure if this is the standard way of passing my own cost estimates. I found this thread in StackOverflow: https://stackoverflow.com/questions/54726015/why-does-apache-calcite-estimates-100-rows-for-all-tables-a-query-contains/54739313#54739313
One of the replies suggests that the only way to inject cardinality estimates for tables is via an ExternalCatalog. I cannot find any information about ExternalCatalog and I am not sure even what it is. I will appreciate if someone guides me or send me some links or examples. Best, Alireza Samadian
