Re: [GENERAL] Enum on-disk format
On ons, 2009-11-18 at 22:33 -0800, Scott Bailey wrote: The only I/O functions I'm aware of are send, recv, in and out. What controls converting from/to wire and on-disk formats? send and recv And why is wire format little endian and disk big endian? The wire format is network order (which is big endian), the disk format is whatever your CPU uses. -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
[GENERAL] Enum on-disk format
I'm trying to better understand the internals of Postgres, and I'm looking at the enum type. The docs say that an enum value is stored on disk as 4 bytes. But enum_send() returns a bytea representing the actual text of the value and not the index of that value. So what step am I missing here? Also, is there a way to see the raw data for the tuple on a page? I was using pageinspect to try to figure out what was happening on the disk. The get_raw_page function returns the entire page and heap_page_items will allow me to find the substring that represents a given tuple. But the rows were much wider than I thought they would be, 28 bytes + 2 byte spacer to store 4 bytes of data. Is there any way to see which bytes of an item pointer actually map to columns in a table? And where can I find more info on how Postgres stores tuples? Scott Bailey -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Enum on-disk format
Scott Bailey arta...@comcast.net writes: I'm trying to better understand the internals of Postgres, and I'm looking at the enum type. The docs say that an enum value is stored on disk as 4 bytes. But enum_send() returns a bytea representing the actual text of the value and not the index of that value. So what step am I missing here? The wire format isn't necessarily the on-disk format. In this case we concluded that the internal OID value wouldn't be of any use to clients. Also, is there a way to see the raw data for the tuple on a page? Try contrib/pageinspect, and read http://developer.postgresql.org/pgdocs/postgres/storage-page-layout.html regards, tom lane -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] Enum on-disk format
Tom Lane wrote:
Scott Bailey arta...@comcast.net writes:
I'm trying to better understand the internals of Postgres, and I'm
looking at the enum type. The docs say that an enum value is stored on
disk as 4 bytes. But enum_send() returns a bytea representing the actual
text of the value and not the index of that value. So what step am I
missing here?
The wire format isn't necessarily the on-disk format. In this case
we concluded that the internal OID value wouldn't be of any use to
clients.
Also, is there a way to see the raw data for the tuple on a page?
Try contrib/pageinspect, and read
http://developer.postgresql.org/pgdocs/postgres/storage-page-layout.html
regards, tom lane
Thanks Tom that did the trick. The only I/O functions I'm aware of are
send, recv, in and out. What controls converting from/to wire and
on-disk formats? And why is wire format little endian and disk big endian?
And for posterity, here's how to get to the raw tuple data.
SELECT substring(page, lp_off + t_hoff + 1, lp_len - t_hoff) AS tuple_data,
sub.*
FROM (
SELECT (heap_page_items(page)).*, page
FROM (
SELECT get_raw_page('test', 0) page
) s
) sub
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