docs: Update TOAST storage docs for configurable compression.

Mention that there are multiple TOAST compression methods and that the
compression method used is stored in a TOAST pointer along with the
other information that was stored there previously. Add a reference to
the documentation for default_toast_compression, where the supported
methods are listed, instead of duplicating that here.

I haven't tried to preserve the text claiming that pglz is "fairly
simple and very fast." I have no view on the veracity of the former
claim, but LZ4 seems to be faster (and to compress better) so it
seems better not to muddy the waters by talking about compression
speed as a strong point of PGLZ.

Patch by me, reviewed by Justin Pryzby.

Discussion: http://postgr.es/m/CA+Tgmoaw_YBwQhOS_hhEPPwFhfAnu+VCLs18EfGr9gQw1z4H-w@mail.gmail.com

diff --git a/doc/src/sgml/storage.sgml b/doc/src/sgml/storage.sgml
index 3234adb639f58b3a4edb00b0e6d58018795b2e5f..bfccda77afde496229e075e3e7012f3d55d25715 100644 (file)
--- a/doc/src/sgml/storage.sgml
+++ b/doc/src/sgml/storage.sgml
@@ -394,9 +394,9 @@ Further details appear in .
 
 
 The compression technique used for either in-line or out-of-line compressed
-data is a fairly simple and very fast member
-of the LZ family of compression techniques.  See
-src/common/pg_lzcompress.c for the details.
+data can be selected using the COMPRESSION option on a per-column
+basis when creating a table. The default for columns with no explicit setting
+is taken from the value of .
 
 
 
@@ -425,8 +425,9 @@ retrieval of the values.  A pointer datum representing an out-of-line on-disk
 TOASTed value therefore needs to store the OID of the
 TOAST table in which to look and the OID of the specific value
 (its chunk_id).  For convenience, pointer datums also store the
-logical datum size (original uncompressed data length) and physical stored size
-(different if compression was applied).  Allowing for the varlena header bytes,
+logical datum size (original uncompressed data length), physical stored size
+(different if compression was applied), and the compression method used, if
+any.  Allowing for the varlena header bytes,
 the total size of an on-disk TOAST pointer datum is therefore 18
 bytes regardless of the actual size of the represented value.