CODEWITHSUNDEEP
sizepostgispointgeography
iloo
iloo

Reputation: 986

Size of data type Geography(Point, 4326) in PostGIS?

Its constructor takes a POINT and a SRID. Since POINT is two doubles (8 bytes each), SRID is an integer (4 bytes), can one assume that the full structure takes 8*2+4=20 bytes of storage space?

Upvotes: 4

Views: 2788

Answers (1)

John Powell
John Powell

Reputation: 12581

Have a look at ST_MemSize. This gives you the size of toast tables, as well, so is more suitable than pg_total_relation_size and other built in Postgres functions, which do not -- although this only applies to larger geometries. Returning to your question, for a point,

SELECT ST_MemSize(ST_MakePoint(0, 0));

returns 32 bytes, which is 4 doubles (rather than the 2 you would expect). The reason for this is that there is additional meta-data, such as the endianness to store.

Add in the SRID, with,

SELECT ST_MemSize(ST_SetSRID(ST_MakePoint(0, 0), 4326));

returns 32 bytes.

Now, a Linestring, with two points,

SELECT ST_MemSize(ST_SetSRID(ST_GeomFromText('LINESTRING(0 0, 1 1)'), 4326));

returns 48 bytes, which is now only 16 bytes, that is 2 doubles, larger than the Point, as you would expect.

Now, if you buffer this point,

SELECT ST_MemSize(ST_SetSRID(ST_Buffer(ST_MakePoint(0, 0), 10), 4326));

you get 568 bytes, which due to the fact that there are 8 points by default per quarter circle segment, plus a repeated start point at the end, which you can check with,

SELECT ST_NPoints(ST_SetSRID(ST_Buffer(ST_MakePoint(0, 0), 10), 4326));

gives you 33*16 = 528, with the other 40 bytes coming from the SRID, endianness, etc.

Upvotes: 11

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