PostgreSQL Crosstab issues / "Return and SQL tuple descriptions are incompatible" - postgresql

Question

Good afternoon, I am using POSTGRESql version 9.2 and I'm trying to use a crosstab function to transpose two columns on a table so that i can later join it to a different SELECT query.
I have installed the tablefunc extension.
However i keep getting this "Return and SQL tuple descriptions are incompatible" error which seems to be because of typecasts.
I don't need them to be a specific type.
My original SELECT query is this
SELECT inventoryid, ttype, tamount
FROM inventorytesting
Which gives me the following result:
inventoryid ttype tamount
2451530088940460 7 0.2
2451530088940460 2 0.5
2451530088940460 8 0.1
2451530088940460 1 15.7
8751530077940461 7 0.7
8751530077940461 2 0.2
8751530077940461 8 1.1
8751530077940461 1 19.2
and my goal is to get it like:
inventoryid 7 2 8 1
8751530077940461 0.7 0.2 1.1 19.2
2451530088940460 0.2 0.5 0.1 15.7
The 'ttype' field has 49 different values such as "7","2","8","1" which are fixed.
The 'tamount' field varies its values depending on the 'inventoryid' field but there will always be 49 of them, even if its value is zero. It will never be "null".
I have tried a few variations that i could find in the internet which sum up to this:
SELECT *
FROM crosstab (
$$SELECT inventoryid, ttype, tamount
FROM inventorytesting
WHERE inventoryid = '2451530088940460'
ORDER BY inventoryid, ttype$$
)
AS ct("inventoryid" text,"ttype" smallint,"tamount" numeric)
The fieldtypes on the inventorytesting table are
select column_name, data_type from information_schema.columns
where table_name = 'inventorytesting'
Results:
column_name data_type
id bigint
ttype smallint
tamount numeric
tunit text
tlessthan smallint
plantid text
sessiontime bigint
deleted smallint
inventoryid text
docdata text
docname text
labid bigint
Any pointers would be great.

Answer 1

demo:db<>fiddle
The resulting table definition has to contain the table structure you are expecting - the pivoted one - and not the structure of the given one:
SELECT *
FROM crosstab(
$$SELECT inventoryid, ttype, tamount
FROM inventorytesting
WHERE inventoryid = '2451530088940460'
ORDER BY inventoryid, ttype$$
)
AS ct("inventoryid" text,"type1" numeric,"type2" numeric,"type7" numeric,"type8" numeric)
Addionally there is no need to use the crosstab function. You can achieve a pivot by simply using the standard CASE function:
SELECT
inventoryid,
SUM(CASE WHEN ttype = 1 THEN tamount END) AS type1,
SUM(CASE WHEN ttype = 2 THEN tamount END) AS type2,
SUM(CASE WHEN ttype = 7 THEN tamount END) AS type7,
SUM(CASE WHEN ttype = 8 THEN tamount END) AS type8
FROM
inventorytesting
GROUP BY 1
If you were on 9.4 or higher you could use the Postgres specific FILTER clause:
SELECT
inventoryid,
SUM(tamount) FILTER (WHERE ttype = 1) AS type1,
SUM(tamount) FILTER (WHERE ttype = 2) AS type2,
SUM(tamount) FILTER (WHERE ttype = 7) AS type7,
SUM(tamount) FILTER (WHERE ttype = 8) AS type8
FROM
inventorytesting
GROUP BY 1
demo:db<>fiddle

Answer 2

With the crosstab, you define the actual result table (basically the result of the pivot). The input query defines three columns which are then processed as:
grouping column result in the actual rows
the pivot columns
value for the pivot column
In your case, the crosstab therefore needs to be defined as:
ct(
"inventoryid" text,
"tamount_1" numeric,
"tamount_2" numeric,
"tamount_3" numeric,
...
)
The column header will then correlate to a certain value of column ttype in the order as defined by the inner query's ORDER BY.
The thing with crosstab is that missing values for ttype (e.g. some value returned for 4 but not for 3), the resulting columns would be 1, 2, 4, ... with 3 being missing. Here, you'd have to make sure (if you need consistent output) that your inner query returns at least a NULL row (e.g. via LEFT JOIN).