> You can't just simply physically re-order columns based on new columns that were added. It seems reasonable to call that "insufficient abstraction". Terminology can vary, but the root comment is pointing out that Postgres didn't abstract away column order and has multiple performance penalties from it. > I wouldn't call an insufficient abstraction It's all just implementation detail, not show-stoppers.
#Postgres rename column update#
I'm just starting with Postgres, thx for reading.ĮDIT: forget this, I just read here the post by "singron" stating "An update in postgres will always copy the tuple for MVCC, so it can't take advantage of an optimization like this to modify it in place", so the row gets moved anyway. I guess that Vacuum would fix such situations (if this actually happens), but maybe I can decrease the work that it has to do. maybe later the more rows are updated the more fragmented the table/tablespace becomes etc.). use space-chars to fill those columns with the expected future length of the string?Īsking because I'm not sure if having the columns contents increase their size will cause kind of "row movement" (row cannot be updated in-place as there is no space to accomodate the new long string) which might(?) entangle a bit the contents of the table (e.g.
10 in a first round, then another 10 in another cycle, then the last 10), is it ok to leave those columns empty or should I e.g. 30 varchar columns which are initially empty but which will be updated later (not all at once but e.g. When I insert rows into a table that has e.g. Thus, I don't see what's wrong with the document – probably I just didn't understand you. in this case, the tuple length now is 32 bytes (24 bytes tuple header + 8 bytes for the data). Test=# select lp_len from heap_page_items(get_raw_page('tttt2', 0)) Test=# create table tttt2 as select 1::int2, 2::int2, 3::int4
#Postgres rename column plus#
If we'd have 2 int2 columns, followed by a int4 one, we'd use just one word (8 bytes to store this data, plus 24 bytes for tuple header): Of course, if the second column is of size 4, then both columns fit into a single word – let's try real + int4: And our int2 column was aligned by 6 additional bytes, to whole 8-byte word. Why 40? Because the tuple header is 23 bytes aligned to 3 "words", hence 24 bytes – so total is 24 + 8 + 8 = 40 though, effective data size us just 23 + 2 + 8 = 33 and 7 bytes are "wasted". the total length of the tuple in this example is 40 bytes. Test=# select lp_len from heap_page_items(get_raw_page('tttt1', 0)) Test=# create table tttt1 as select 1::int2 as c1, 2::int8 as c2 For example, with smalling (which has size 2 bytes but might be aligned, depending on the "column tetris" situation we have) + int8: 1 word is 8 bytes, and if you use two columns, the first one having size 2 bytes, and the second – 8 bytes, you'll end up spending 8 + 8 = 16 bytes, because the first one (2byte in length) will be aligned with 6 zeroes to fill the whole "8-byte word".
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