From 5861f9c303c2ac6707af6c5292ad38a66cfb658d Mon Sep 17 00:00:00 2001 From: Amit Kapila Date: Wed, 20 Feb 2019 16:10:31 +0530 Subject: [PATCH] Doc: Update the documentation for FSM behavior for small tables. In commit b0eaa4c51b, we have avoided the creation of FSM for small tables. So the functions that use FSM to compute the free space can return zero for such tables. This was previously also possible for the cases where the vacuum has not been triggered to update FSM. This commit updates the comments in code and documentation to reflect this behavior. Author: John Naylor Reviewed-by: Amit Kapila Discussion: https://postgr.es/m/CACPNZCtba-3m1q3A8gxA_vxg=T7gQf7gMbpR4Ciy5LntY-j+0Q@mail.gmail.com --- contrib/pgstattuple/pgstatapprox.c | 3 +++ doc/src/sgml/pgfreespacemap.sgml | 2 ++ doc/src/sgml/pgstattuple.sgml | 4 +++- 3 files changed, 8 insertions(+), 1 deletion(-) diff --git a/contrib/pgstattuple/pgstatapprox.c b/contrib/pgstattuple/pgstatapprox.c index ff7c255..d36758a 100644 --- a/contrib/pgstattuple/pgstatapprox.c +++ b/contrib/pgstattuple/pgstatapprox.c @@ -89,6 +89,9 @@ statapprox_heap(Relation rel, output_type *stat) /* * If the page has only visible tuples, then we can find out the free * space from the FSM and move on. + * + * Note: If a relation has no FSM, GetRecordedFreeSpace() will report + * zero free space. This is fine for the purposes of approximation. */ if (VM_ALL_VISIBLE(rel, blkno, &vmbuffer)) { diff --git a/doc/src/sgml/pgfreespacemap.sgml b/doc/src/sgml/pgfreespacemap.sgml index 0122d27..0bcf79b 100644 --- a/doc/src/sgml/pgfreespacemap.sgml +++ b/doc/src/sgml/pgfreespacemap.sgml @@ -61,6 +61,8 @@ The values stored in the free space map are not exact. They're rounded to precision of 1/256th of BLCKSZ (32 bytes with default BLCKSZ), and they're not kept fully up-to-date as tuples are inserted and updated. + In addition, small tables don't have a free space map, so these functions + will return zero even if free space is available. diff --git a/doc/src/sgml/pgstattuple.sgml b/doc/src/sgml/pgstattuple.sgml index b17b3c5..8797131 100644 --- a/doc/src/sgml/pgstattuple.sgml +++ b/doc/src/sgml/pgstattuple.sgml @@ -527,7 +527,9 @@ approx_free_percent | 2.09 bit set, then it is assumed to contain no dead tuples). For such pages, it derives the free space value from the free space map, and assumes that the rest of the space on the page is taken up by live - tuples. + tuples. Small tables don't have a free space map, so in that case + this function will report zero free space, likewise inflating the + approximate tuple length. -- 1.8.3.1