Crosstab with a large or undefined number of categories

Question

My real problem has to do with recording which of a very large number of anti-virus products agree that a given sample is a member of a given anti-virus family. The databas

Answer 1

Your wish implies transfering some of the data (the names) into column headings, i.e. the schema of the resulting table. As this is somewhere between inconvenient and impossible, I would recommending just sorting and summing the data in sql, and doing the rest outside of the database.

SELECT Photo, Voter
FROM data
WHERE Decision = '...'
ORDER BY Photo, Voter

and

SELECT Photo, COUNT(*) AS Total
FROM data
WHERE Decision = '...'
GROUP BY Photo
ORDER BY Photo;

Answer 2

Using the same sample data as Clodoaldo ("create table vote...") and using the plpythonu function make_pivot_table (below), you can run:

create temp table pivot_data on commit drop as 
    select * from vote where decision = 'Cat' union select photo, null, null from vote;

select * from make_pivot_table('{photo}', 'voter',  'decision', 'count', 'pivot_data',
  'pivot_result', false);

select * from pivot_result order by photo;

The make_pivot_table function definition is:

-- make_pivot_table
-- python version 0.9
-- last edited 2015-08-11 

create or replace function
 make_pivot_table(row_headers text[], category_field text, value_field text,
  value_action text, input_table text, output_table text, keep_result boolean)
returns void as
$$
# imports
from collections import defaultdict
import operator
import string

# constants
BATCH_SIZE = 100
VALID_ACTIONS = ('count', 'sum', 'min', 'max')
NULL_CATEGORY_NAME = 'NULL_CATEGORY'
TOTAL_COL = 'total'

# functions
def table_exists(tablename):
    plan = plpy.prepare("""select table_schema, table_name from
        information_schema.Tables where table_schema not in ('information_schema',
        'pg_catalog') and table_name = $1""", ["text"])
    rows = plpy.execute(plan, [input_table], 2)
    return bool(rows)

def make_rowkey(row):
    return tuple([row[header] for header in row_headers])

def quote_if_needed(value):
    return plpy.quote_literal(value) if isinstance(value, basestring) else str(value)

# assumes None is never a value in the dct
def update_if(dct, key, new_value, op, result=True):
    current_value = dct.get(key)
    if current_value is None or op(value, current_value) == result:
        dct[key] = new_value

def update_output_table(output_table, row_headers, colname, value):
    pg_value = plpy.quote_literal(value) if isinstance(value, basestring) else value
    sql = 'update %s set %s = %s where ' % (output_table, plpy.quote_ident(colname), 
                                            pg_value)
    conditions = []
    for index, row_header in enumerate(row_headers):
        conditions.append('%s = %s' % (plpy.quote_ident(row_header),
                                       quote_if_needed(rowkey[index])))
    sql += ' and '.join(conditions)
    plpy.execute(sql)


# -----------------

if not table_exists(input_table):
    plpy.error('input_table %s dones not exist' % input_table)

if value_action not in VALID_ACTIONS:
    plpy.error('%s is not a recognised action' % value_action)

# load the data into a dict
count_dict = defaultdict(int)
sum_dict = defaultdict(float)
total_dict = defaultdict(float)
min_dict = dict()
max_dict = dict()
categories_seen = set()
rowkeys_seen = set()
do_total = value_action in ('count', 'sum')

cursor = plpy.cursor('select * from %s' % plpy.quote_ident(input_table))
while True:
    rows = cursor.fetch(BATCH_SIZE)
    if not rows:
        break
    for row in rows:
        rowkey = make_rowkey(row)
        rowkeys_seen.add(rowkey)
        category = row[category_field]           
        value = row[value_field]
        dctkey = (rowkey, category)

        # skip if value field is null
        if value is None:
            continue

        categories_seen.add(category)

        if value_action == 'count':
        count_dict[dctkey] += 1
        total_dict[rowkey] += 1
    if value_action == 'sum':
            sum_dict[dctkey] += value
            total_dict[rowkey] += value
        if value_action == 'min':
            update_if(min_dict, dctkey, value, operator.lt)
        if value_action == 'max':
            update_if(max_dict, dctkey, value, operator.gt)

plpy.notice('seen %s summary rows and %s categories' % (len(rowkeys_seen),
                                                        len(categories_seen)))

# get the columns types
coltype_dict = dict()
input_type_sql = 'select * from %s where false' % plpy.quote_ident(input_table)
input_type_result = plpy.execute(input_type_sql)
for index, colname in enumerate(input_type_result.colnames()):
    coltype_num = input_type_result.coltypes()[index]
    coltype_sql = 'select typname from pg_type where oid = %s' % coltype_num
    coltype = list(plpy.cursor(coltype_sql))[0]
    plpy.notice('%s: %s' % (colname, coltype['typname']))
    coltype_dict[colname] = coltype['typname']

plpy.execute('drop table if exists %s' % plpy.quote_ident(output_table))
sql_parts = []
if keep_result:
    sql_parts.append('create table %s (' % plpy.quote_ident(output_table))
else:
    sql_parts.append('create temp table %s (' % plpy.quote_ident(output_table))

cols = []
for row_header in row_headers:
    cols.append('%s %s' % (plpy.quote_ident(row_header), coltype_dict[row_header]))

cat_type = 'bigint' if value_action == 'count' else coltype_dict[value_field]

for col in sorted(categories_seen):
    if col is None:
        cols.append('%s %s' % (plpy.quote_ident(NULL_CATEGORY_NAME), cat_type))
    else:
        cols.append('%s %s' % (plpy.quote_ident(col), cat_type))

if do_total:
    cols.append('%s %s' % (TOTAL_COL, cat_type))

sql_parts.append(',\n'.join(cols))
if keep_result:
    sql_parts.append(')')
else:
    sql_parts.append(') on commit drop')
plpy.execute('\n'.join(sql_parts))

dict_map = {'count': count_dict, 'sum': sum_dict, 'min': min_dict, 'max': max_dict }
value_dict = dict_map[value_action]
for rowkey in rowkeys_seen:
    sql = 'insert into %s values (' % plpy.quote_ident(output_table)
    sql += ', '.join([quote_if_needed(part) for part in rowkey])
    sql += ')'
    plpy.execute(sql)

if do_total:
    for rowkey, value in total_dict.iteritems():
        update_output_table(output_table, row_headers, TOTAL_COL, value)

for (rowkey, category), value in value_dict.iteritems():
    # put in cateogry value
    colname = NULL_CATEGORY_NAME if category is None else category
    update_output_table(output_table, row_headers, colname, value)

$$ language plpythonu

Answer 3

create table vote (Photo integer, Voter text, Decision text);
insert into vote values
(1, 'Alex', 'Cat'),
(1, 'Bob', 'Dog'),
(1, 'Carol', 'Cat'),
(1, 'Dave', 'Cat'),
(1, 'Ed', 'Cat'),
(2, 'Alex', 'Cat'),
(2, 'Bob', 'Dog'),
(2, 'Carol', 'Cat'),
(2, 'Dave', 'Cat'),
(2, 'Ed', 'Dog'),
(3, 'Alex', 'Horse'),
(3, 'Bob', 'Horse'),
(3, 'Carol', 'Dog'),
(3, 'Dave', 'Horse'),
(3, 'Ed', 'Horse'),
(4, 'Alex', 'Horse'),
(4, 'Bob', 'Horse'),
(4, 'Carol', 'Cat'),
(4, 'Dave', 'Horse'),
(4, 'Ed', 'Horse'),
(5, 'Alex', 'Dog'),
(5, 'Bob', 'Cat'),
(5, 'Carol', 'Cat'),
(5, 'Dave', 'Cat'),
(5, 'Ed', 'Cat')
;

The query for the cat:

select photo,
    alex + bob + carol + dave + ed as Total,
    alex, bob, carol, dave, ed
from crosstab($$
    select
        photo, voter,
        case decision when 'Cat' then 1 else 0 end
    from vote
    order by photo
    $$,'
    select distinct voter
    from vote
    order by voter
    '
) as (
    photo integer,
    Alex integer,
    Bob integer,
    Carol integer,
    Dave integer,
    Ed integer
);
 photo | total | alex | bob | carol | dave | ed 
-------+-------+------+-----+-------+------+----
     1 |     4 |    1 |   0 |     1 |    1 |  1
     2 |     3 |    1 |   0 |     1 |    1 |  0
     3 |     0 |    0 |   0 |     0 |    0 |  0
     4 |     1 |    0 |   0 |     1 |    0 |  0
     5 |     4 |    0 |   1 |     1 |    1 |  1

If the number of voters is big or not known then it can be done dynamically:

do $do$
declare
voter_list text;
r record;
begin

drop table if exists pivot;

voter_list := (
    select string_agg(distinct voter, ' ' order by voter) from vote
    );

execute(format('
    create table pivot (
        decision text,
        photo integer,
        Total integer,
        %1$s
    )', (replace(voter_list, ' ', ' integer, ') || ' integer')
));

for r in
select distinct decision from vote
loop
    execute (format($f$
        insert into pivot
        select
            %3$L as decision,
            photo,
            %1$s as Total,
            %2$s
        from crosstab($ct$
            select
                photo, voter,
                case decision when %3$L then 1 else 0 end
            from vote
            order by photo
            $ct$,$ct$
            select distinct voter
            from vote
            order by voter
            $ct$
        ) as (
            photo integer,
            %4$s
        );$f$,
        replace(voter_list, ' ', ' + '),
        replace(voter_list, ' ', ', '),
        r.decision,
        replace(voter_list, ' ', ' integer, ') || ' integer'
    ));
end loop;
end; $do$;

The above code created the table pivot with all the decisions:

select * from pivot where decision = 'Cat';