As it says on the Oracle XE overview page:
Oracle Database XE can be installed on any size host machine with any number of CPUs (one database per machine), but XE will store up to 4GB of user data, use up to 1GB of memory, and use one CPU on the host machine.
Now if I want to monitor the database to see how much user data is in use or how much memory is the database using, how would I do that? It is possible to monitor these values from Oracle Application Express, but I want to monitor the database from a centralized monitoring system. Oracle XE documentation presents a query which returns Flash Recovery Area usage, so I'm guessing there's a similar query for user data usage also.
SELECT
NAME,
TO_CHAR(SPACE_LIMIT, '999,999,999,999') AS SPACE_LIMIT,
TO_CHAR(SPACE_LIMIT - SPACE_USED + SPACE_RECLAIMABLE,'999,999,999,999')
AS SPACE_AVAILABLE,
ROUND((SPACE_USED - SPACE_RECLAIMABLE)/SPACE_LIMIT * 100, 1)
AS PERCENT_FULL
FROM V$RECOVERY_FILE_DEST;
Also, what happens when user data is above the limit?
Shamelessly taken from the Oracle FAQ website, here is a query that checks used space by tablespace:
SELECT /* + RULE */ df.tablespace_name "Tablespace",
df.bytes / (1024 * 1024) "Size (MB)",
SUM(fs.bytes) / (1024 * 1024) "Free (MB)",
Nvl(Round(SUM(fs.bytes) * 100 / df.bytes),1) "% Free",
Round((df.bytes - SUM(fs.bytes)) * 100 / df.bytes) "% Used"
FROM dba_free_space fs,
(SELECT tablespace_name,SUM(bytes) bytes
FROM dba_data_files
GROUP BY tablespace_name) df
WHERE fs.tablespace_name (+) = df.tablespace_name
GROUP BY df.tablespace_name,df.bytes
UNION ALL
SELECT /* + RULE */ df.tablespace_name tspace,
fs.bytes / (1024 * 1024),
SUM(df.bytes_free) / (1024 * 1024),
Nvl(Round((SUM(fs.bytes) - df.bytes_used) * 100 / fs.bytes), 1),
Round((SUM(fs.bytes) - df.bytes_free) * 100 / fs.bytes)
FROM dba_temp_files fs,
(SELECT tablespace_name,bytes_free,bytes_used
FROM v$temp_space_header
GROUP BY tablespace_name,bytes_free,bytes_used) df
WHERE fs.tablespace_name (+) = df.tablespace_name
GROUP BY df.tablespace_name,fs.bytes,df.bytes_free,df.bytes_used
ORDER BY 4 DESC;
By default, user data is the space used in the USERS tablespace.
As for what happens when you are at the limit, I can only guess that: