问题
Do you know what the maximum number of columns that can be queried in Postgresql? I need to know this before I start my project.
回答1:
According to About PostgreSQL it's "250 - 1600 depending on column types". See "Limits". The column types affect it because in PostgreSQL rows may be at most 8kb (one page) wide, they cannot span pages. Big values in columns are OK because TOAST handles that, but there's a limit to how many columns you can fit in that depends on how wide the un-TOASTed data types used are.
(Strictly this refers to columns that can be stored in on-disk rows; queries might be able to use wider column sets than this. I don't recommend relying on it.)
If you're even thinking about approaching the column limits you're probably going to have issues.
Mapping spreadsheets to relational databases seems like the simplest thing in the world - map columns to columns, rows to rows, and go. Right? In reality, spreadsheets are huge freeform monsters that enforce no structure and can be really unweildy. Relational databases are designed to handle lots more rows, but at a cost; in the case of PostgreSQL part of that cost is a limitation to how wide it likes those rows to be. When facing spreadsheets created by Joe User this can be a real problem.
One "solution" is to decompose them into EAV, but that's unspeakably slow and ugly to work with. Better solutions are using arrays where possible, composite types, hstore, json, xml, etc.
In the end, though, sometimes the best answer is to analyse the spreadsheet using a spreadsheet.
回答2:
For others who might find this information useful the answer is 1663 depending on the types of columns occording to this post http://archives.postgresql.org/pgsql-admin/2008-05/msg00208.php
回答3:
One reason to have a lot of columns is to store a large word vector, from text mining.
Text mining can produce 10,000 or more features, aka columns.
You could use a data warehouse like MonetDB, which claims that "the number of columns per tables is practically unlimited".
回答4:
With the use of the JSON/JSONB type, there almost is no need to have very many columns in a table.
And in rare cases, if you were to hit the maximum column limit of your database system, maybe you could use an RDBMS implementation of a spreadsheet, with a table like the following:
create table wide_table(
id serial not null primary key
,rownum integer not null
,colnum integer not null
,colname varchar(30) not null
,coltype varchar(30) not null
,nullable boolean not null
,collen integer
,colprec integer
,colscale integer
,colvalue raw(2000)
,unique (rownum,colnum)
);
This would allow for practically unlimited number of columns, but using it would be less trivial.
来源：https://stackoverflow.com/questions/12606842/what-is-the-maximum-number-of-columns-in-a-postgresql-select-query