Sunday, May 17, 2009

Data Structures in Python and SQL

This is -- partially -- about the object-relational impedance mismatch.  But it's also about the parallel concepts between objects and relations.  We'll use Python as our object model.

First, the obvious.

A SQL table is a list of rows.  A row is a dictionary that maps a column name to a column value.  A SQL table has a defined type for a named column; Python doesn't pre-define the type of each column.

Some folks like to think of a table as a rigidly-defined class, which is partly true.  It can be rigidly-defined.  However, the extra meta-data doesn't help much.


As a practical matter, most databases go beyond the minimalist definition of a relationship as a collection of rows.  An index extends the structure in one of two ways.

A unique-key index transforms the SQL table into a dictionary that maps a key to a row.
    class UniqueKeyTable( object ):
        def __init__( self ):
            self.rows = {}
        def insert( self, aRow ):
            self.rows[aRow.key()]= [aRow]
The non-unique key index transforms the SQL table into a dictionary that maps a key to a list of rows.
    class KeyedTable( object ):
def __init__( self ):
self.rows = collections.defaultdict(list)
def insert( self, aRow ):
self.rows[aRow.key()].append( aRow )
SQL Operations

The single-table SELECT algorithm has a WHERE clause that gets broken into two parts: key filtering and everything else.

The basic SELECT looks something like this.
    for k in table.rows[key]:
        for r in table.rows[k]:
if other_where_clause( r ):
             select_group_by( r )
That's the essential feature of a basic select -- it expresses a number of design patterns.  There's a key-to-list map, a filter, and the "select-group-by" map to results.

In theory, the SELECT operation is the more general "filter" algorithm, where every row passes through the a general where_clause_filter process.  

The Join Algorithms

We have a number of alternative join algorithms.  In some cases, we have two dictionaries with the same keys.  This leads to a highly optimized query where one key locates rows on both sides of the join.

In other cases, we have a kind of nested-loops join.  We find a row in one table, and use this row's attributes to locate a row in another table.

The "Which is Better?" Question

We always have two alternatives for every algorithm:  the SQL version and the Python version.  This is an essential issue in resolving the Object-Relational Impedance mismatch issue.  We can implement our algorithm on either side: Python objects or SQL relations.

Note that there's no simple "Use SQL for this" or "Use Python for that" decision process.  The two structures -- objects and relations -- are completely isomorphic.  There's no specific set of features that dominate either representation.  

The literal question that I got was "Should I use a complex data structure in a programming language or should I use SQL ?"

Ideally, the answer is "SQL does [X] better", leading to an easy decision.  But this kind of answer doesn't exist.

The two structures are isomorphic; the correct answer is hard to determine.  You want the RDBMS to filter rows and return the smallest relevant set of data to the object representation.  While locating the fewest rows seems simple, a few things make even this hard to determine.  

While it seems that the RDBMS can be the best way to handle join algorithms, this doesn't always work.  When we're doing a join involving small tables, the RDBMS may be less effective than an in-memory dictionary.  It sometimes occurs that SQL is best for filtering very large tables only.

Indeed, the only way to chose among two isomorphic representations (objects vs. relations) is to benchmark each implementation.

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