Functions are the main mechanism with which extensions are implemented. The are three kinds of functions:

  • Row functions
  • Aggregate functions
  • Virtual tables

madIS also provides syntactic extensions, in its supported SQL, that simplify the use of above functions.

Row functions

Programmable row functions work in a similar way as standard SQL row functions such as abs(), lower() and upper(). Their implementation is done in Python, and they are able to use all Python facilities and libraries.

An example of an SQL statement using an external row function is:

mterm> select detectlang('Il en est des livres comme du feu de nos foyers');

Above statement executes the detectlang function, which tries to detect the language of a snippet of text by analysing its statistical properties. On the above example the snippet is a Voltaire quote, and the correct answer from detectlang is that it is in french.

If we wished to do the same for multiple quotes then we could use the following SQL statement (assuming the existence of a quotes_table which contains the column quote):

mterm> select detectlang(quote) from quotes_table;

In general external row functions can be used in exactly the same way as SQLite’s internal row functions.

Creating and using a simple row function

Python files that implement row functions live in “/src/functions/row”. A single python file can contain many row functions. Now lets create a simple “hello world” row function.

Go to the “src/functions/row/” path of madIS, and create “”. Then write the following code in it:

def hello(*args):
    return "hello "+args[0]


The code above, looks like and indeed is, a simple python function that takes multiple arguments. The only peculiarity is the “hello.registered=True” line. What this line does, is to instruct madIS to register this function with the SQLite core. This way, Python functions that we don’t wish to become SQLite row functions, can simply be used without any change to them.

To execute the above row function we start madIS and execute an SQL query which uses the new hello row function:

% python
mterm> select hello("world");
hello world

The SQL query above, shows the results of calling hello function with “world” as a parameter. As happens very often in SQL queries, parameter values are retrieved from tables. Below we show such an example:

mterm> select "world" as a UNION select "all" as a;
Query executed in 0 min. 0 sec 3 msec
mterm> select hello(a) from (select "world" as a UNION select "all" as a);
hello all
hello world
Query executed in 0 min. 0 sec 7 msec

When developing custom row functions it is very useful to browse through the already developed row functions, in the “functions/row” path of madIS. Most of the time a custom function can be based on an already existing one.


If a developer wishes to add a row function that in essence is a part of a pre-existing Python library. The madIS convention is to put the library files in the “src/lib/” path of madIS and then create a row function in the “row” path of madIS which imports and calls the necessary methods of the actual library in “lib” path. For an example of this see detectlang.

Aggregate functions

Programmable aggregate functions work in a similar way as standard SQL aggregate functions such as sum(), min() and max(). Their implementation is done in Python, and as is also the case for row functions, all Python facilities and libraries are available.

An example of an SQL statement using an external aggregate function is:

mterm> select "term1+term2" as a UNION select "term2 term3" as a;
term2 term3
mterm> select concatterms(a)
from (select "term1+term2" as a UNION select "term2 term3" as a);
term1+term2 term2 term3

The statement above, executes the concatterms() function, which concatenates strings of terms together, while keeping the terms disjoint.

Using the aggregate function above, together with a row function is also possible. To concatenate together only the input string keywords we could execute the following SQL:

mterm> select concatterms(keywords(a))
         from (select "term1+term2" as a UNION select "term2 term3" as a);
term1 term2 term2 term3

Creating and using a simple aggregate function

Python files that implement aggregate functions live in “/src/functions/aggregate”. A single python file can contain many aggregate functions. Now, lets create a simple aggregate function.

Go to the “src/functions/aggregate/” path of madIS, and create “”. Then write the following code in it:

class quotedconcatterms:


   def __init__(self):

   def step(self, *args):
       if len(args[0])!=0:

   def final(self):
       return ' '.join(self.result)

What the aggregate function above does, is to concatenate strings of terms together while inserting single quotes around each string.

In the “__init__” method of class quotedconcatterms we initialize all needed variables (self.result). The “step” method, is called by SQLite’s engine for every tuple of the group which is fed into the aggregate function. At the end of the group, the “final” method is called to return the final result.

The “registered=True” line above, as in row functions, instructs madIS to load the class into the SQLite engine as an aggregate function.

To execute the above aggregate function we start madIS and execute an SQL query which uses the new quotedconcatterms aggregate function (notice the single quotes around each input string in the results):

% python
mterm> select quotedconcatterms(keywords(a)) from (select "term1+term2" as a UNION select "term2 term3" as a);
'term1 term2' 'term2 term3'

As is the case with aggregate functions it is very useful to browse through the already developed aggregate functions, in the “functions/aggregate” path of madIS, to find snippets of code implementing wished for, functionality.

Virtual tables

Virtual tables are actually functions that take parameters and output table like data. They can be used in the SQL syntax wherever a regular table would be placed.

Virtual tables are one of the most powerful function classes in madIS. They can function in a regular table fashion, where the output data is finite in number, or in a streaming fashion, where the output stream can be infinite.

This, together with the SQL syntax extensions of madIS, creates a very powerful environment with which a variety of standard madIS features have been developed, such as the multisets, workflow engine, direct data load from filesystem or network, etc.

Typical examples of virtual tables sources are files, SQL query resulsets, or even external programs output.

In addition to the above, virtual tables can also function as programmable indexes on the input data, negotiating with the database engine about which constraints or ordering they’ll be able to answer/produce quickly. The only madIS virtual table that implements and works as an index is cache, which offers, a created on the fly, multidimensional index.

Using virtual tables can be done in a variety of ways. The first is parametric table:

% python
mterm> select * from file('./demo/continents.tsv') limit 2;

What the example above does, is to read the tab separated “continents.tsv” filesystem file as a table. Using the standard limit SQL directive, the output is limited to only 2 rows.

An example showing the streaming nature of virtual tables is:

mterm> select C1 from pipe('query:date');
Fri Feb 1 12:34:55 EET 2000
mterm> select strsplit(C1,'delimiter: ') from pipe('query:date');

The first of the above examples executes system’s “date” command through the use of the pipe virtual table. Notice that the pipe virtual table has a default returned schema, having one column named C1.

The second of the above examples does the same as the previous one, however it also splits pipe’s output, using space as a delimiter, into multiple columns through strsplit() row function.

At this point, we should skip a little ahead, and mention two peculiarities on the queries above. The first one is the “query:date” parameter of pipe. This in madIS is called a named parameter, where the part before the “:” is a parameter’s name and the second part is a parameter’s value.

The second peculiarity is the that the row function strsplit() returns multiple columns. This is a multiset function, able to return as many columns and rows as it wishes. To see the column names and column types of second query above, we can execute:

mterm> coltypes select strsplit(C1,'delimiter: ') from pipe('query:date');

The “coltypes” in the above query may look like a special function of the terminal but in essence it is also a virtual table that takes as input a query and returns a table having as first column the column names of the inside query and as second column the column types of the inside query.


  • coltypes is able to be used in front of a query because it is in its inverted form. We’ll discuss madIS SQL extensions in the following section.
  • We won’t present a simple virtual table implementation as they are too complex to be presented here. For more see the already existing tables in the “/src/functions/vtable” path of madIS, and the APSW documentation concerning virtual tables.

madIS SQL extensions

While developing madIS, we realized that some simple ideas like virtual table chaining, were too difficult to be written in plain SQL. In addition using a virtual table was more difficult than it needed to be, due to having to “create” a virtual table before using it.

To solve the problems above, we’ve extended SQL with the following features:

  • Parametric virtual tables are automatically created and deleted
  • Syntax inversion

The first of above features, is quite apparent. The user doesn’t have to manage virtual tables, madIS does everything it is needed automatically in the background, creating a virtual table just before a query using it, is executed, and deleting it after the query’s execution completes. This is why we could directly read from the file system, with the file virtual table, in the previous section.

Inversions are slightly more complex. So lets start with a simple virtual table chaining example and invert it:

mterm> select * from output('d.txt', "query:select * from pipe('query:date')");

What the query above does, is to call system’s “date” command, through the pipe virtual table, and write the output back to the file system (in the file named “d.txt”). The output virtual table returns back if it succeeded or not (1 or 0).

Because chaining virtual tables on the manner shown above, proved to require a lot of effort and in addition was error prone, syntax inversion was added to madIS’ accepted SQL syntax. The inversion syntax is:

VTname 'param1' 'param2'... namedparam1:valueofnamedparam1 ... SQL_query

Whenever madIS’ SQL preprocessor “sees” above syntax it turns it into:

select * from Vtname('param1', 'param2' ..., 'namedparam1:valueofnamedparam1' ... , 'query:SQL_query')

So lets use the inverted syntax, to reissue the query above:

mterm> output 'd.txt' select * from pipe('query:date');

In practice a wide range of functionalities has been possible with inversions. The coltypes example shown in the previous section “feels” like special functionality that is implemented execution engine in other DB systems. In madIS, most of the special commands are in essence virtual tables (or row functions) in their inverted form.

The same inverted syntax is also offered by madIS, for row functions. So instead of:

mterm> select detectlang('Il en est des livres comme du feu de nos foyers');

We could have written it as such:

mterm> detectlang 'Il en est des livres comme du feu de nos foyers';

A problem arises with row inversions. How to distinguish between the case of passing the actual query to a row function, and passing the results of the query to the row function.

madIS has the var row function, which sets and retrieves a madIS variable. Some examples of its usage:

mterm> var 'demo' 'time'; -- This puts value 'time' to the variable named 'demo'
mterm> var 'demo'; -- This retrieves the value of variable 'demo'
mterm> select var('demo','time');--The same queries in their not inverted forms
mterm> select var('demo');

What would happen if instead of the value “time” we had a plain query?:

mterm> var 'demo2' select var('demo');
select var('demo')
mterm> var 'demo2';
select var('demo')

We see above, that instead of putting the result of the query “select var(‘demo’)” into “demo2”, it put the query’s text in it. This makes it complex to pass values between variables, forcing us to do:

mterm> select var('demo2', (select var('demo'))); --Non inverted syntax
mterm> var 'demo2';

Due to this, a directive which forces the use of the results of the embedded query has been created, to ease use cases such as above:

mterm> var 'demo2' 'essence of';
essence of
mterm> var 'demo' 'time';
mterm> var 'demo2' FROM var 'demo';
mterm> var 'demo2';

Notice the “FROM” keyword after the “var ‘demo2’” part of the query. What “FROM” does is to transform the query in its non inverted syntax shown above.


Both “FROM” and “OF” are accepted as synonyms, when specifying the use of embedded query results.


Early in the development of madIS, the need to return multiple rows and columns from row and aggregate functions, arose. This need was satisfied through the use of expand virtual table and an accompanying Python API (CompBuffer) to create multisets.

Virtual table expand works by looking into the results of a SQL query, and if it finds a specially formatted tuple (in CompBuffer’s format), it decompresses it, looks for schema information, and then starts returning one by one the compressed data to the outside query. If more than one CompBuffers are found, then their schemata are consolidated into one.

CompBuffer tries to avoid writing on the hard disk, by compressing in memory, the data fed into it and only after a threshold of 10 Mb of compressed data is reached, spilling over into the hard disk.

To make multiset usage transparent to the user, madIS’ SQL preprocessor, inserts automatically the expand virtual table whenever a multiset producing function is met. To flag a row or aggregate function as a multiset one, the programmer has to set the function’s attribute “multiset” to “True”. This is done in the same way as for the “registered” function’s attribute.

An example of a multiset row function which produces multiple columns is:

mterm> select strsplit("one,two,three");

And one which produces multiple rows is:

mterm> select strsplitv("one,two,three");

Both of the functions above, break their input on “comma” by default.

On the fly multidimensional indexing (the cache virtual table)

Working with external sources directly, has the major problem of not being able to put acceleration structures on them (indexing), unless materializing them in the form of a database table. Joins with external sources, in particular, are very slow, forcing the SQLite core to do multiple sequential passes on the virtual tables tied to the external sources.

To solve the problems above, the cache virtual table was created. The way it works is this:

  • At the beginning it does one pass over the results of the input query, and keeps them in memory
  • Then it negotiates with SQLite’s core about what constraints and order bys, the core would like to have answered as fast as possible
  • It builds a very fast multidimensional index based on the negotiations above
  • It waits until the core starts sending queries on the above constraints, and returns the results using the just build multidimensional index
  • At the end of the query it throws away the cached query’s data and the multidimensional index

The multidimensional index is based on kd-trees and is implemented in Python, using the virtual table indexing API. It is a very fast to be build index, and extremely fast on answering queries involving many constraints. The multidimensional index is also designed to be memory efficient, needing O(number_of_dataset_rows) memory no matter the number of indexed dimensions.

In practice, it has been up to 10 times faster on interval joins compared to SQLite’s native indexing, and as the dimensions grow in number its speed advantage becomes bigger.

An example of cache use is:

% cd demo
% python
mterm> select country.c2, continent.c1
  from file('countries.tsv') as country,
       file('continents.tsv') as continent
  where country.c1=continent.c2;
Antigua and Barbuda|Americas
United Arab Emirates|Asia
. . . . . . . . .
Query executed in 0 min 2 sec 40 msec
mterm> select country.c2, continent.c1
  from file('countries.tsv') as country,
       (CACHE file 'continents.tsv') as continent
  where country.c1=continent.c2;
Antigua and Barbuda|Americas
United Arab Emirates|Asia
. . . . . . . . .
Query executed in 0 min 0 sec 71 msec

The queries above, do a direct join on the filesystem’s files “countries.tsv” and “continents.tsv” (in the “demo” path of madIS). The first query doesn’t do any caching at all, so it is forced to do multiple sequential reads of the “continents” file, to execute the join. The second query, using the inverted syntax, caches the “continents” file and creates the required indexes to execute the join quickly.

The speed difference between the two queries is quite significant (cached version of query is 28x faster). For bigger datasets and more constraints the speed advantage could be even greater.

See also


From the beginning madIS was designed for building complete data processing systems with it. In madIS, workflows are viewed as a series of queries to be executed sequentially.

The main function that implements madIS’ workflow engine is exec virtual table:

mterm> select 'select 5';
select 5
mterm> exec select 'select 5';

Virtual table exec, takes as input an SQL query, executes it and then operates on the results of the input query, executing them sequentially and returning “1” if all the queries executed successfully, or throwing an error if any queries failed to execute correctly.

This peculiar design (its a meta-meta function), enables exec to work in tandem with other virtual tables that feed to it the workflows to be executed. The most frequent combination is:

mterm> exec flow file 'workflow.sql';

What the query above does, is to open and read the “workflow.sql” filesystem file via the file virtual table, then feed its results into the flow virtual table which collects in one line the “broken into multiple lines” queries, and finally to feed the resulting queries to exec virtual table which will execute them.

The above combination of virtual tables presents, one the most powerful aspects of madIS. Namely the ability to create complex functionality (in this instance a workflow engine), by quickly combining simple entities.


Another benefit of using (or abusing) a relational database to implement a workflow engine, is that all ACID properties of the database are still present during the workflow processing. Unfortunately due to limitations of the SQLite core, transactional execution of madIS workflows isn’t fully working yet (see Limitations of madIS).

See also

Designing and building a data processing system with madIS

Concerning the design of a data processing system. The required steps that have to be made when designing a data processing system with madIS are:

  1. Define the problem you are trying to solve
  2. Break the problem into workflows
  3. Break the workflows into queries
  4. Find the functions that the queries will need
  5. Find the Python libraries that the functions will need

The steps above, are mostly self evident, and should be made in the order that they are presented.

To build the data processing system, the above steps have to be followed, finding/building the required entities (functions, queries), for each step, going from bottom to top (Step 5 to Step 2),

In practice we have found that there is always some overlap between the designing and building stages and between steps. Great care should be given, when designing, on the most difficult and time consuming step, which is step 1. When step 1 is successfully executed, the speed with which the rest of the steps are designed and following that, building them from bottom to top, is extremely fast in our experience.


An example of a madIS workflow can be found in the examples section.

Limitations of madIS

The limitations of madIS are:

  • Due to SQLite’s inability to simultaneously use multiple CPUs to execute a query, madIS’ query execution is done by only one CPU. This will be partially solved in the future through the use of the map virtual table, which will be able to execute a row function on multiple data rows in parallel. For more heavy processing concurrency, functions and virtual tables enabling madIS to be used on clusters are in the planning stage
  • Due to SQLite’s locking semantics, workflows inside a parent workflow, aren’t in their own transactions. So when a child workflow fails, the whole parent transaction rolls back. This will be solved whenever SQLite fixes the problem where the database’s system schema table is locked whenever a table is deleted