Is there some limit on the maximum total amount of data that can be stored in a single table in Oracle?
I think there shouldn't be because tables are anyways stored as a set of rows and rows can be chained as well. Does such a limit exist?
See Physical Database Limits and Logical Database Limits documentation.
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Why does Heroku have a row limit on their Hobby plan if there is already an overall database size limit? I'm confused because I've reached my row limit, but I'm nowhere near the size limit. Does the amount of rows you store affect what it costs for them to manage it or is that cost only affected by the amount of bytes in your data?
Edit: Also, what constitutes a row, because I added 50 items to a table but it only added one row to my row limit? I thought each item you add to a table is a "row" on the table.
It is to stop people using custom data types to store more than 1 row worth of info in a single row. They want to limit the amount of data people can store so they limit the number of rows, but to do this without limiting row size they also need an overall size limit.
The Heroku Postgres dev plan will be limited to 10,000 rows. Not limit had been enforced. This is a global limit.
I somehow want to calculate partition overhead for very small tables. I have a Oracle DB with 5K tables sizing from lets say 10KB to 1TB. All of them are range partitioned based on a DATE column. What I want to calculate is the difference in the table size if I will store all data in 1 partition Vs if I store it in lets say 30 partitions. Block Size is 16KB.
My question is, in Oracle database if there is a 5Gb table and the SGA size is 10GB, if I select the 5Gb table, it will fit into 10GB SGA size.
But if my table is more than 10GB and my SGA size is 5Gb, then how do select queries work, does it display all the rows of 10GB table, and how does the buffer CACHE works?
If we have a table which is of more size than than the SGA, then we will have to define the size of SGA again to the required size, and then the problem is solved.
we can define the size of SGA and PGA while creating the database.
Buffered Cache Working:
When we request data from disk, at minimum oracle reads one block.Even if we request only one row, many rows in the same table are likely to be retrieved, which lie in the same block. The same goes for column.
A block in the buffered cache can be in one of the three states:
Free: Currently not used.
Pinned: Currently being accessed.
Dirty: Block has been modified but currently not been written to disk.
A block write up is triggered when one of the following happens:
The database is issued a shutdown command.
A full or partial checkpoint occurs.
A recovery time threshold occurs, which is again set by us.
A free block is needed and none are found after a given amount of time( we use LRU algorithm here).
Certain Data Definition Language commands(DDL).
Every three seconds. There are many other reasons, the algorithm is complex and can change with each release of oracle.
I am adding a column with datatype varchar2(1000), This column will be used to store a large set of message(approximately (600 characters).Does it effect the performance of query for having large datatype length, if so how? I will be having a query selecting that column occasionally. Does a table consume extra memory here even if the value in that field in some places 100 characters?
Does it affect performance? It depends.
If "adding a column" implies that you have an existing table with existing data that you're adding a new column to, are you going to populate the new column for old data? If so, depending on your PCTFREE settings and the existing size of the rows, increasing the size of every row by an average of 600 bytes could well lead to row migration which could potentially increase the amount of I/O that queries need to perform to fetch a row. You may want to create a new table with the new column and move the old data to the new table while simultaneously populating the new column if this is a concern.
If you have queries that involve full table scans on the table, anything that you do that increases the size of the table will negatively impact the speed of those queries since they now have to read more data.
When you increase the size of a row, you decrease the number of rows per block. That would tend to increase the pressure on your buffer cache so you'd either be caching fewer rows from this table or you'd be aging out some other blocks faster. Either of those could lead to individual queries doing more physical I/O rather than logical I/O and thus running longer.
A VARCHAR2(1000) will only use whatever space is actually required to store a particular value. If some rows only need 100 bytes, Oracle would only allocate 100 bytes within the block. If other rows need 900 bytes, Oracle would allocate 900 bytes within the block.
As part of maintenance, we remove a considerable amount of records from one table and wish to dispose of this space for other table. The thing is that I'm checking the size of the table and it shows the same original size before the delete. What am I missing here?
To check the size, I'm looking at the dba_segments.
There are some concepts you need to know about tables before you can really understand space usage. I will try and give you the 5 second tour ...
A table is made up of extents, these can vary in size, but lets just assume they are 1MB chunks here.
When you create a table, normally 1 extent is added to it, so even though the table has no rows, it will occupy 1MB.
When you insert rows, Oracle has a internal pointer for the table known as the high water mark (HWM). Below the HWM, there are formatted data block, and above it there are unformated blocks. It starts at zero.
If you take a new table as an example, and start inserting rows, the HWM will move up, giving more and more of that first extent to be used by the table. When the extent is all used up, another one will be allocated and the process repeats.
Lets says you fill a three 1MB extents and then delete all the rows - the table is empty, but Oracle does not move the HWM back down, or free those used extents. So the table will take 3MB on disk even though it is empty, but there is 3MB of free space to be reused in it. New inserts will go into that space below the HWM until it is filled up again before the HWM is move again.
To recover the space, if your table is using ASSM, you can use the command:
alter table t1 shrink space;
If you are not using ASSM, the you need to think about a table reorg to recover the space.
If you want to "reclaim" the space, the easiest method is:
ALTER TABLE table MOVE TABLESPACE different_tablespace;
ALTER TABLE table MOVE TABLESPACE original_tablespace;
Providing you have:
Some downtime in which to do it
A second tablespace with enough space to transfer the table into.
Take a look at this site
about the table size after deleting rows.
Space is effectively reused when you delete. Your database will not show any new free
space in dba_free_space -- it will have more blocks on freelists and more empty holes in
index structures.
SELECT SUM(BYTES)
FROM DBA_SEGMENTS
WHERE SEGMENT_NAME LIKE 'YOUR TABLE NAME%';
You will get the right answer.