Why do parallel scans require syncscans (but not really)?

Hi,

While investigating something, I noticed it's not really possible to
disable syncscans for a particular parallel scan.

That is, with serial scans, we can do this:

table_index_build_scan(heap, index, indexInfo, false, true,
callback, state, NULL);

where false means "allow_sync=false", i.e. disabling synchronized scans.

However, if this is tied to a parallel scan, that's not possible. If you
try doing this:

table_index_build_scan(heap, index, indexInfo, false, true,
callback, state, parallel_scan);

it hits this code in heapam_index_build_range_scan()

/*
* Parallel index build.
*
* Parallel case never registers/unregisters own snapshot. Snapshot
* is taken from parallel heap scan, and is SnapshotAny or an MVCC
* snapshot, based on same criteria as serial case.
*/
Assert(!IsBootstrapProcessingMode());
Assert(allow_sync);
snapshot = scan->rs_snapshot;

Sadly, there's no explanation why parallel scans do not allow disabling
sync scans just like serial scans - and it's not quite obvious to me.

Just removing the assert is not sufficient to make this work, though.
Because just a little later we call heap_setscanlimits() which does:

Assert(!scan->rs_inited); /* else too late to change */
/* else rs_startblock is significant */
Assert(!(scan->rs_base.rs_flags & SO_ALLOW_SYNC));

I don't quite understand what the comments for the asserts say, but
SO_ALLOW_SYNC seems to come from table_beginscan_parallel.

Perhaps the goal is to prevent mismatch between the parallel and serial
parts of the scans - OK, that probably makes sense. But I still don't
understand why table_beginscan_parallel forces the SO_ALLOW_SYNC instead
of having an allow_sync parameters too.

Is there a reason why all parallel plans should / must be synchronized?

Well, in fact they are not *required* because if I set

synchronize_seqscans=off

this makes the initscan() to remove the SO_ALLOW_SYNC ...

regards

--
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
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