If I am correct, SQLite stores a database per file, and a file can't store more than one databases.
How does PostgreSQL store a database in terms of file(s)? Does it also store a database per file, and a file can't store more than one databases?
(SQLite uses more than one file for the rollback journal or when in WAL mode.)
The PostgreSQL database file layout is documented in its documentation:
Each table and index is stored in a separate file. For ordinary relations, these files are named after the table or index's filenode number, which can be found in pg_class.relfilenode. […] in addition to the main file (a/k/a main fork), each table and index has a free space map …, which stores information about free space available in the relation. The free space map is stored in a file named with the filenode number plus the suffix _fsm. Tables also have a visibility map, stored in a fork with the suffix _vm, to track which pages are known to have no dead tuples. […]
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I tried to look into data dictionary of postgresql for long time
but i didn't help.
I'm trying to know what state is the data file in, is it available for the db instance or not
photo with example
I tryed to look into pg_class table
Postgres does not have the concept of "online" (or "offline") data files.
When you create a table it creates a file for that table. For every GB of data, Postgres will add a new file. If you drop the table all files belonging to that table are removed.
So the files for a table are always "online".
For more details on how Postgres stores the data, I recommend reading the chapter Database Physical Storage in the manual.
Or read The Internals of PostgreSQL - Chapter 1: Database Cluster, Databases, and Tables
I'm referring to file organization in DBMS. But I can't understand what is Blocking Records. If you can please explain me the term Blocking Records.
Blocking record is, File records divide into Blocks. A database is a collection of large amount of related data. In case of RDBMS (Relational Database Management System), the data is stored in the form of relations or tables.
when stored the data stored in tables but actually this huge amount of data is stored in the form of files in physical memory.(A File is a collection of related records stored on the secondary storage)
There are various strategies for mapping file records into blocks of disk
1. Spanned Mapping -: suppose when File record is too large. hence stored inside the block even if it can only be stored partially hence we stored a record of file in two blocks.
2. Unspanned Mapping -: the record of a file is stored inside the block only if it can be stored completely inside it.
What is the hierarchy of Database related objects in postgres SQL?
Should it be like, table space must be created at instance level unlike other RDBMS(where we have table space under database).
If so we create the table space at instance level, what is the purpose of database? and what is difference between table space and database on postgres server?
An instance (in PostgreSQL called cluster) is a data directory initialized with initdb with a PostgreSQL server process.
A tablespace is a directory outside the data directory where objects can also be stored. Tablespaces are useful for certain corner cases like distributing I/O or limiting space for a subset of the data.
A database is a container for objects with permissions, organized in schemas.
The difference is that tablespaces are a physical concept, it defines a space where the data are stored, while databases are a logical concept about how data are organized, what they mean, how they are related, who is allowed to access them and so on.
The two concepts are orthogonal.
A database can have tables in several tablespaces, and a tablespace can contain data from several databases.
Database is where you organize all your objects. Tablespace is just storage space for those object.
You can storage your db object in different Tablespace. For example one table is storage in a Tablespace in diskA but another Table use a Tablespace in diskB to improve the performance. Or maybe you need a tablespace for big tables and dont mind use a slow big HDD for those objects.
I'm interested to get the physical locations of tables, views, functions, data/content available in the tables of PostgreSQL in Linux OS. I've a scenario that PostgreSQL could be installed in SD-Card facility and Hard-Disk. If I've tables, views, functions, data in SD, I want to get the physical locations of the same and merge/copy into my hard-disk whenever I wish to replace the storage space. I hope the storage of database should be in terms of plain files architecture.
Also, is it possible to view the contents of the files? I mean, can I access them?
Kevin and Mike already provided pointers where to find the data directory. For the physical location of a table in the file system, use:
SELECT pg_relation_filepath('my_table');
Don't mess with the files directly unless you know exactly what you are doing.
A database as a whole is represented by a subdirectory in PGDATA/base:
If you use tablespaces it gets more complicated. Read details in the chapter Database File Layout in the manual:
For each database in the cluster there is a subdirectory within
PGDATA/base, named after the database's OID in pg_database. This
subdirectory is the default location for the database's files; in
particular, its system catalogs are stored there.
...
Each table and index is stored in a separate file. For ordinary
relations, these files are named after the table or index's filenode
number, which can be found in pg_class.relfilenode.
...
The pg_relation_filepath() function shows the entire path (relative to
PGDATA) of any relation.
Bold emphasis mine.
The manual about the function pg_relation_filepath().
The query show data_directory; will show you the main data directory. But that doesn't necessarily tell you where things are stored.
PostgreSQL lets you define new tablespaces. A tablespace is a named directory in the filesystem. PostgreSQL lets you store individual tables, indexes, and entire databases in any permissible tablespace. So if a database were created in a specific tablespace, I believe none of its objects would appear in the data directory.
For solid run-time information about where things are stored on disk, you'll probably need to query pg_database, pg_tablespace, or pg_tables from the system catalogs. Tablespace information might also be available in the information_schema views.
But for merging or copying to your hard disk, using these files is almost certainly a Bad Thing. For that kind of work, pg_dump is your friend.
If you're talking about copying the disk files as a form of backup, you should probably read this, especially the section on Continuous Archiving and Point-in-Time Recovery (PITR):
http://www.postgresql.org/docs/current/interactive/backup.html
If you're thinking about trying to directly access and interpret data in the disk files, bypassing the database management system, that is a very bad idea for a lot of reasons. For one, the storage scheme is very complex. For another, it tends to change in every new major release (issued once per year). Thirdly, the ghost of E.F. Codd will probably haunt you; see rules 8, 9, 11, and 12 of Codd's 12 rules.
I have little doubt, I have a field of type oid in my database I'm saving the text files, I wonder if I can somehow retrieve the name with which you saved the file. Do not know if in some way to do so successfully *lo_export* or another method.
Thanks in advance for the help.
PostgreSQL does not create a separate file for each large object stored in the database; a fairly normal heap table with btree index is used. So large objects are broken into smaller pieces for easier space management by the database, and intermingled within the 1GB segment files used by that table.
Taking a quick peek at the pg_class table, I see an entry for pg_largeobject, which I think is where all objects stored with the "lo" large object feature are stored. On my system I see a relfilenode of 11869, which means that the initial file for storing data would be 11869 and subsequent files would be 11869.1, 11869.2, etc. I don't know whether there is any way for the relfilenode to be reassigned for large objects, but you should probably check your pg_class entry to be sure.
Generally, large objects stored in the database should not be accessed except through the "lo" functions provided. If you want separate files and the ability to access them directly, you should probably save them directly to disk and store the filename or a URI. You could save them to disk from inside a PostgreSQL function, or externally.