A separate table level option to control compression

Hello,

Currently either the table level option `toast_tuple_target` or the compile
time default `TOAST_TUPLE_TARGET` is used to decide whether a new tuple
should be compressed or not. While this works reasonably well for most
situations, at times the user may not want to pay the overhead of toasting,
yet take benefits of inline compression.

I would like to propose a new table level option, compress_tuple_target,
which can be set independently of toast_tuple_target, and is checked while
deciding whether to compress the new tuple or not.

For example,

CREATE TABLE compresstest250 (a int, b text) WITH (compress_tuple_target =
250);
CREATE TABLE compresstest2040 (a int, b text) WITH (compress_tuple_target =
2040);

-- shouldn't get compressed nor toasted
INSERT INTO compresstest250 VALUES (1, repeat('1234567890',20));

-- should get compressed, but not toasted
INSERT INTO compresstest250 VALUES (2, repeat('1234567890',30));

-- shouldn't get compressed nor toasted
INSERT INTO compresstest2040 VALUES (1, repeat('1234567890',20));
INSERT INTO compresstest2040 VALUES (2, repeat('1234567890',30));

Without this patch, the second INSERT will not compress the tuple since its
length is less than the toast threshold. With the patch and after setting
table level option, one can compress such tuples.

The attached patch implements this idea.

Thanks,
Pavan

--
Pavan Deolasee http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services