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https://issues.apache.org/jira/browse/DERBY-712?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12712573#action_12712573
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Suran Jayathilaka commented on DERBY-712:
-----------------------------------------
Following are some questions that I asked Rick Hillegas and Rick's answers to
them. Posting here because the conversation wasn't sent to the mailing list,
which I will do henceforth.
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1. Once I have the barebones classes for the SEQUENCE catalog, what's the best
way to get about testing them and fleshing them out?
A: It's usually easiest to write the test after you've done the parser changes
for CREATE/DROP'ing your schema objects. Right now, you could add an upgrade
test to verify that the new catalog is created after hard-upgrade but not after
soft-upgrade. The upgrade tests live in
trunk/java/testing/org/apache/derbyTesting/functionTests/tests/upgradeTests/
2. How can I define what comes in the indexColumnPositions array in the
SYSSEQUENCESRowFactory?
I currently have
private static final int[][] indexColumnPositions =
{
{SYSSEQUENCES_SEQUENCEID},
{SYSSEQUENCES_SEQUENCENAME, SYSSEQUENCES_SCHEMAID}
};
A: That looks like a good start. That is the array which defines the number and
shapes of the indexes defined on your catalog. As you get into coding support
for compilation and dependency tracking, you may discover that you need to
define some other indexes to speed up access to the sequence descriptors. You
can add more indexes later if you need them, so you don't have to worry too
much about this right now...
3. What's the use of the "uniqueness" array?
A: The array is null if ALL of the indexes on the catalog are unique indexes.
If any of the indexes is non-unique, then you need to fill in an array of
booleans, one per index. The slot in that array is true if the corresponding
index is unique, and false otherwise. See for instance SYSCOLPERMSRowFactory.
Right now both of your indexes are unique so you can leave the uniqueness array
as null.
4. In StoredFormatIds, I see 2 fields for some elements like TRIGGERS
i.e.
/**
class org.apache.derby.impl.sql.catalog.TriggerDescriptorFinder
*/
static public final int TRIGGER_DESCRIPTOR_FINDER_V01_ID = (MIN_ID_2 +
320);
/**
class org.apache.derby.impl.sql.catalog.TriggerDescriptorFinder
*/
static public final int TRIGGER_DESCRIPTOR_V01_ID = (MIN_ID_2 + 316);
What's the difference in usage of these 2 IDs? And why is there not a class
named TriggerDescriptorFinder as mentioned in the comments here?
A: I think that at one point there was a DependableFinder for every kind of
TupleDescriptor. The DependableFinders are objects which are serialized into
the DEPENDENTFINDER and PROVIDERFINDER columns of SYSDEPENDS. When they are
deserialized, they are capable of asking the data dictionary for the correct
kind of TupleDescriptors for the corresponding UUID columns in SYSDEPENDS. This
is how persistent dependencies are serialized and how they reconstruct the
dependency graph when they are read in.
At some point most of the DependableFinders were collapsed into a single class,
DDdependableFinder and some tricky (de)serialization logic was added so that
the serialized form of the DependableFinders could be as simple as possible,
namely a format id corresponding to the appropriate kind of TupleDescriptor.
Because triggers can invoke sequence generators, you will have to use
SYSDEPENDS to record persistent dependencies between triggers and sequences. So
you will need to add a DependableFinder format id as well as a format id for
the SequenceDescriptor.
5. In the Authorizer class, do I need to add fields for
creating/modifying/dropping sequences and perms, or are they covered in SCHEMA
privileges?
A: Creating/dropping sequences should behave like the creating/dropping of
other schema objects: If you own the schema, then you implicitly have
permission to create/drop any kind of schema object inside that schema. So no
new privileges are needed just to create/drop sequences.
To invoke a sequence, however, a new privilege is needed: USAGE. This is
like EXECUTE_PRIV or INSERT_PRIV.
6. Where in the code do the catalog tables get created when creating a new
database?
A: That happens in DataDictionaryImpl.createDictionaryTables().
7. Do I need to add entries to the C_NodeTypes class? I didn't fully understand
the purpose of this class from the javadoc comments.
A: This class was introduced in order to eliminate a large NodeFactory
interface. That was a worthy goal, but I think that as a result, node creation
has been made weakly typed and hard to follow. Each kind of node has its own
corresponding node type. When you get to the parser work, you will need to add
a CreateSequenceNode, a DropSequenceNode, and a NextValue node--each with its
own node type in C_NodeTypes.
> Support for sequences
> ---------------------
>
> Key: DERBY-712
> URL: https://issues.apache.org/jira/browse/DERBY-712
> Project: Derby
> Issue Type: New Feature
> Components: SQL
> Environment: feature request
> Reporter: Tony Dahbura
> Assignee: Suran Jayathilaka
> Attachments: SequenceGenerator.html
>
>
> Would like to see support added for sequences. This would permit a select
> against the sequence to always obtain a ever increasing/decreasing value.
> The identity column works fine but there are times for applications where the
> application needs to obtain the sequence number and use it prior to the
> database write. Subsequent calls to the table/column would result in a new
> number on each call.
> SQL such as the following:
> SELECT NEXT VALUE FOR sequence_name FROM sometable ; would result in a next
> value.
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