获取PostgreSQL查询中大对象的大小？ [英] Get size of large object in PostgreSQL query?

问题描述

我想获取blob的字节大小。

I would like to obtain the byte size of a blob.

我正在使用Postgresql，并希望通过SQL查询获取大小。像这样：

I am using Postgresql and would like to obtain the size using an SQL query. Something like this:

SELECT sizeof(field) FROM table;

在Postgresql中可能吗？

Is this possible in Postgresql?

更新：我已经阅读了postgresql手册，找不到合适的函数来计算文件大小。另外，blob存储为大对象。

Update: I have read the postgresql manual and could not find an appropriate function to calculate the file size. Also, the blob is stored as a large object.

推荐答案

不是我使用过大对象，而是查看文档： http://www.postgresql.org/docs/current/interactive/lo -interfaces.html＃LO-TELL

Not that I've used large objects, but looking at the docs: http://www.postgresql.org/docs/current/interactive/lo-interfaces.html#LO-TELL

我认为您必须使用某些文件系统API要求的相同技术：搜寻到最后，然后告诉位置。 PostgreSQL的SQL函数似乎包装了内部C函数。我找不到太多文档，但这行得通：

I think you have to use the same technique as some file system APIs require: seek to the end, then tell the position. PostgreSQL has SQL functions that appear to wrap the internal C functions. I couldn't find much documentation, but this worked:

CREATE OR REPLACE FUNCTION get_lo_size(oid) RETURNS bigint
VOLATILE STRICT
LANGUAGE 'plpgsql'
AS $$
DECLARE
    fd integer;
    sz bigint;
BEGIN
    -- Open the LO; N.B. it needs to be in a transaction otherwise it will close immediately.
    -- Luckily a function invocation makes its own transaction if necessary.
    -- The mode x'40000'::int corresponds to the PostgreSQL LO mode INV_READ = 0x40000.
    fd := lo_open($1, x'40000'::int);
    -- Seek to the end.  2 = SEEK_END.
    PERFORM lo_lseek(fd, 0, 2);
    -- Fetch the current file position; since we're at the end, this is the size.
    sz := lo_tell(fd);
    -- Remember to close it, since the function may be called as part of a larger transaction.
    PERFORM lo_close(fd);
    -- Return the size.
    RETURN sz;
END;
$$; 

测试它：

-- Make a new LO, returns an OID e.g. 1234567
SELECT lo_create(0);

-- Populate it with data somehow
...

-- Get the length.
SELECT get_lo_size(1234567);

LO功能似乎主要是通过客户端或通过低级服务器编程使用的，但至少他们为其提供了一些SQL可见函数，这使得上述操作成为可能。我对pg_proc的 SELECT relname进行了查询，其中的relname喜欢'lo％'以使自己入门。 C语言编程的模糊记忆以及对 x'40000':: int 和 SEEK_END = 2 模式的一些研究剩下的就需要值！

It seems the LO functionality is designed to be used mostly through the client or through low-level server programming, but at least they've provided some SQL visible functions for it, which makes the above possible. I did a query for SELECT relname FROM pg_proc where relname LIKE 'lo%' to get myself started. Vague memories of C programming and a bit of research for the mode x'40000'::int and SEEK_END = 2 value were needed for the rest!

这篇关于获取PostgreSQL查询中大对象的大小？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！