While designing writeable CTEs, I found it needs a new infrastructure that helps to share the result data among separated PlannedStmt.
Up to now, in a portal we only have multiple plans that share nothing but relations, then executor starts, runs and ends separately. Writeable CTEs need the result tuplestore of separated plans that were executed before main query . In a prototype Marko and I made adds a List of DerivedTable to Portal, which holds each tuplestore and tupledesc set and the previously-executed result of the child plans are stored in these structure. Here we have some problems: - the process to make tuplestore receiver, to set it to executor of child plans, to run it and to update snapshot is very common but very local in pquery.c. We need similar copy in explain.c and functions.c - we are not sure where to save this List of DerivedTable. In pquery.c it is Portal, and we can see it in ExplainState in explan.c. But very messy... - Wait, isn't there any problem to share data without relations among separated plans? The most conservative way to solve it is to make it by rewriter not by planner to separated executions with temporary tables instead of tuplestore. In this way we don't do so much in planner and writeable CTEs is only a rewrite process. The first reason why we are hacking the planner not rewriter is that the main purpose of writeable CTEs is the performance gained by using tuplestore instead of temporary tables. In the second place, we will be able to extend it to combine RECURSIVE queries (which may be discarded in the first version for simplicity). Please find attached patch (pquery.c only) as a prototype we are working to see what is done about the problem. PlannedStmt now has childStmt member to store separated child plans(PlannedStmts) and the portal calls FillDerivedTables() to execute them and fill tuplestores. Tuplestores are saved in the portal member ddts. They will be referred by main query so they are passed to CreateQueryDesc() on the main query execution. I'm very sorry the explanation is poor, but if you have some thoughts, please let us know. Regards, -- Hitoshi Harada
wcte_pquery_experimental.20101107.patch
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