Combine cmin and cmax fields of HeapTupleHeaders into a single field, by
keeping private state in each backend that has inserted and deleted the same tuple during its current top-level transaction. This is sufficient since there is no need to be able to determine the cmin/cmax from any other transaction. This gets us back down to 23-byte headers, removing a penalty paid in 8.0 to support subtransactions. Patch by Heikki Linnakangas, with minor revisions by moi, following a design hashed out awhile back on the pghackers list.
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src/backend/utils/time/combocid.c
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src/include/utils/combocid.h
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src/test/regress/sql/combocid.sql
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