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- Resolution: Unresolved
- Fix Version/s: None
Currently, if someone has a null key in a map, HCatInputFormat will terminate with an NPE while trying to read it.
This is because we use a TreeMap to preserve order of elements in the map when reading from the underlying storage/serde.
This problem is easily fixed in a number of ways:
a) Switch to HashMap, which allows null keys. That does not preserve order of keys, which should not be important for map fields, but if we desire that, we have a solution for that too - LinkedHashMap, which would both retain order and allow us to insert null keys into the map.
b) Ignore null keyed entries - check if the field we read is null, and if it is, then ignore that item in the record altogether. This way, HCat is robust in what it does - it does not terminate with an NPE, and it does not allow null keys in maps that might be problematic to layers above us that are not used to seeing nulls as keys in maps.
Why do I bring up the second fix? First, I bring it up because of the way we discovered this bug. When reading from an RCFile, we do not notice this bug. If the same query that produced the RCFile instead produces an Orcfile, and we try reading from it, we see this problem.
RCFile seems to be quietly stripping any null key entries, whereas Orc retains them. This is why we didn't notice this problem for a long while, and suddenly, now, we are. Now, if we fix our code to allow nulls in map keys through to layers above, we expose layers above to this change, which may then cause them to break. (Technically, this is stretching the case because we already break now if they care) More importantly, though, we have a case now, where the same data will be exposed differently if it were stored as orc or if it were stored as rcfile. And as a layer that is supposed to make storage invisible to the end user, HCat should attempt to provide some consistency in how data behaves to the end user.
Secondly, whether or not nulls should be supported as keys in Maps seems to be almost a religious view. Some people see it from a perspective of a 'mapping', which lends itself to a 'Sure, if we encounter a null, we map to this other value' kind of a view, whereas other people view it from a 'lookup index' kind of view, which lends itself to a 'null as a key makes no sense - What kind of lookup do you expect to perform?' kind of view. Both views have their points, and it makes sense to see if we need to support it.
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There is another important concern at hand here: nulls in map keys might be due to bad data(corruption or loading error), and by stripping them, we might be silently hiding that from the user. So 'silent stripping' is bad. This is an important point that does steer me towards the former approach, of passing it on to layers above, and standardize on an understanding that null keys in maps are acceptable data that layers above us have to handle. After that, it could be taken on as a further consistency fix, to fix RCFile so that it allows nulls in map keys.
Having gone through this discussion of standardization, another important question is whether or not there is actually a use-case for null keys in maps in data. If there isn't, maybe we shouldn't allow writing that in the first place, and both orc and rcfile must simply error out to the end user if they try to write a null map key? Well, it is true that it is possible that data errors lead to null keys, but it's also possible that the user wants to store a mapping for value transformations, and they might have a transformation for null as well. In the case I encountered it, they were writing out an intermediate table after having read from a sparse table using a custom input format that generated an arbitrary number of columns, and were using the map to store column name mappings that would eventually be written out to another table. That seems a valid use, and we shouldn't prevent users from this sort of usage.
Another reason for not allowing null keys from a java perspective is locking and concurrency concerns, where locking on a null is a pain, per disagreements between Joshua Bloch and Doug Lea in the design of HashMap and ConcurrentHashMap. However, given that HCatalog reads are happening in a thread on a drone where there should be no parallel access of that record, and more importantly, this should strictly be used in a read-only kind of usage, we should not have to worry about that.
Increasingly, my preference is to change to LinkedHashMaps to allow null keys, and for consistency's sake, after this is tackled, to see if we should be fixing RCFile to allow null keys(this might be trickier since RCFile has a lot of other users that are probably currently working.)
Another option is to change to LinkedHashMap, but also add a conf key to hcat to allow the user to specify whether or not we want to strip nulls. That way, a user can specify what behaviour they like. That's more cruft though, and I don't want to go down that path unless there is a user that explicitly wants/needs that.
Anyone have any other thoughts on the matter?
HIVE-11625Map instances with null keys are not properly handled for Parquet tables
HIVE-11713Map instances with null keys are not written unless table file format is ORC.
HIVE-7282HCatLoader fail to load Orc map with null key
HIVE-7282HCatLoader fail to load Orc map with null key
- Sushanth Sowmyan
- Sushanth Sowmyan
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A hive in the Windows Registry is the name given to a major section of the registry that contains registry keys, registry subkeys, and registry values.
All keys that are considered hives begin with 'HKEY' and are at the root, or the top of the hierarchy in the registry, which is why they're also sometimes called root keys or core system hives.
Where Are the Registry Hives Located?
In Registry Editor, the hives are the set of registry keys that appear as folders on the left-hand side of the screen when all other keys have been minimized.
Here is a list of the common registry hives in Windows:
HKEY_DYN_DATA is a registry hive that was used in Windows ME, 98, and 95. Most of the information stored in that hive is stored in HKEY_LOCAL_MACHINEHARDWARE in later versions of Windows.
Why Can't I See Any Registry Hives?
Sometimes, when you open Registry Editor, you'll see lots and lots of folders on the left side, and maybe even registry values on the right side, but not any registry hives. This just means that the registry hives are out of the normal viewing area.
To see all of the registry hives at once, scroll to the very top of the left side of the Registry Editor and collapse all of the hives, either by tapping or clicking the down arrows or choosing Collapse from the right-click menu.
Either way, this will minimize all of the keys and subkeys so you just see the handful of registry hives listed above.
Another reason some registry hives aren't showing up is if you're viewing the registry remotely from a different computer. Follow that link for more information on why Windows Registry doesn't look like it normally does in that context.
Registry Hive vs Registry Key
A registry hive is a folder in the Windows Registry, but so is a registry key. So what exactly is the difference between a registry hive and registry key?
The only difference between the two is that a registry hive is the first folder in the registry, and it contains registry keys, whereas the registry keys are the folders inside the hives that contain registry values and other registry keys.
Naming a folder in the registry a 'registry hive' is only done to further categorize what it is that we're talking about. Instead of calling every folder in the registry a registry hive or a registry key, we call the major, first folder a hive but use key as the name of every other folder inside of the hives, and registry subkeys as the term for keys that exist within other keys.
A Registry Hive in Context
Here is an easy way to understand where a registry hive belongs in the Windows Registry:
Microsoft word with serial key. As you can see in the example below, while there may be multiple registry subkeys below a hive, there's always only just one registry hive within each location.
- HIVE: HKEY_CURRENT_USER
- KEY: Control Panel
- SUBKEY: Desktop
- SUBKEY: Colors
- VALUE: Menu
Editing and Deleting Registry Hives
Registry hives, unlike registry keys and values, can not be created, deleted, or renamed. Registry Editor won't let you, meaning you can't even accidentally edit a registry hive.
Reading Hive For Serial Keys
Being unable to remove registry hives isn't Microsoft keeping you from doing something amazing with your own computer - there's simply no reason you'd ever want to. The keys and values that comprise all the registry hives are where the real value of the Windows Registry is.
Backing Up Registry Hives
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You can, however, back up registry hives, just like you can registry keys. Backing up an entire hive saves all of the keys and values within that hive as a REG file that can then be imported back into the Windows Registry at a later time.
See How to Back Up the Windows Registry, and the accompanying How to Restore the Windows Registry, for more.