Always enter parallel mode when scanning a table in parallel #604
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Previously we were only calling `EnterParallelMode()` if we were not already in parallel mode. And then when we were done with that the scan for which we entered parallel mode, we would call `ExitParallelMode()`. This had the problem that if we were doing two scans, we could call ExitParallelMode() before both scans were done: 1. Scan A starts: EnterParallelMode 2. Scan B starts: do nothing 3. Scan A exists: ExitParallelMode 4. Scan B is now running in parallel even though we exited parallel mode. This was causing an assertion in the Postgres code to fail. This change fixes that by always calling `EnterParallelMode()` when we do a parallel scan and always calling `ExitParallelMode()` when we are finished with a scan. This works because Postgres internally keeps a counter of the amount of times we entered parallel mode. So now in the above example we would do: 1. Scan A starts: EnterParallelMode(), counter 1, enter parallel mode 1. Scan B starts: EnterParallelMode(), counter 2, do nothing 3. Scan A exists: ExitParallelMode(), counter 1, do nothing 3. Scan B exists: ExitParallelMode(), counter 0, exit parallel mode
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Previously we were only calling
EnterParallelMode()if we were not already in parallel mode. And then when we were done with that the scan for which we entered parallel mode, we would callExitParallelMode(). This had the problem that if we were doing two scans, we could call ExitParallelMode() before both scans were done:This was causing an assertion in the Postgres code to fail.
This change fixes that by always calling
EnterParallelMode()when we do a parallel scan and always callingExitParallelMode()when we are finished with a scan. This works because Postgres internally keeps a counter of the amount of times we entered parallel mode. So now in the above example we would do:Fixes #592