- Motivation
A regular B-tree index provides efficient mapping of key values to tuples within a table. However, if you have two tables connected in some way, a regular B-tree index may not be efficient enough. In this case, you would need to create an index for each table. The purpose will become clearer if we consider a simple example which is the main use-case as I see it.
- Example
We need to store a graph. So we create a table for nodes
CREATE TABLE Nodes (
id SERIAL PRIMARY KEY,
label VARCHAR(255)
);
and a table for edges
CREATE TABLE Edges (
label VARCHAR(255),
source INTEGER REFERENCES Nodes(id),
target INTEGER REFERENCES Nodes(id)
);
In order to efficiently traverse a graph we would have an index for Nodes.id which created automatically in this case and an index for Edges.source.
- Tweaked B-Tree
We could optimize cases like the former by modifying PostgreSQL btree index to allow it to index 2 tables simultaneously.
Semantically it would be UNIQUE index for attribute x of table A and an index for attribute y in table B. In the non-deduplicated case index tuple pointing to a tuple in A should be marked by a flag. In the deduplicated case first TID in an array can be interpreted as a link to A.
During the vacuum of A an index tuple pointing to a dead tuple in A should be cleaned as well as all index tuples for the same key. We can reach this by clearing all index tuples after the dead one until we come to index tuple marked by a flag with different key. Or we can enforce deduplication in such index.
In the example with a graph such index would provide PRIMARY KEY for Nodes and the index for Edges.Source. The query
SELECT * FROM Nodes WHERE id = X;
will use this index and take into account only a TID in Nodes (so this would be marked index tuple or first TID in a posting list). The query
SELECT * FROM Edges WHERE source = X;
conversely will ignore links to Nodes.
-- Syntax
I believe that
CREATE TABLE Nodes (
id SERIAL PRIMARY KEY ADJACENT,
label VARCHAR(255)
);
CREATE TABLE Edges (
label VARCHAR(255),
source INTEGER REFERENCES ADJACENT Nodes(id),
target INTEGER REFERENCES Nodes(id)
);
would suffice for this new semantics.
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