Ancient History Mark (AHM) Clarifications
In the Vertica Knowldge base , In link "https://community.dev.hpe.com/t5/Vertica-Knowledge-Base/Understanding-Vertica-Epochs/ta-p/233749", I got explanation of AHM as
"A large epoch map can increase the catalog size. The ancient history mark is the epoch prior to which historical data can be purged from physical storage.You cannot run any historical queries prior to the AHM. By default, Vertica advances the AHM at an interval of 5 minutes to be equal with LGE.".
I need clarification for the below set of items.
1) A large epoch map can increase the catalog size. What is epoch map, is there any catolog table for this ?
2) Explanation says that "You cannot run any historical queries prior to the AHM", what historical queries exactly means, is it only the retriving the DELETE data or some thing else ?
3) As per definition it says AHM will be moved to LGE at an interval 5 mins, if so the maximum retention of deleted data in Vertica is just 5 mins ?
4) If the above point is valid then what is the significae of the below parameters in relationship with AHM,
HistoryRetentionTime,HistoryRetentionEpochs and PurgeMergeoutPercent. Kindly elobarate