指定した期間のみのデータを集計したい

postgreの仕様がよくわからないですがYEAR関数的なものはないのでしょうか？

create table tbl(id int,create_datetime datetime);
insert into tbl values
(1,'2016-01-01 00:00:00'),
(2,'2016-12-01 00:00:00'),
(3,'2016-12-31 23:59:59'),
(4,'2017-01-01 00:00:00');

バルク処理ができない場合は、適当にデータ投入してください。

ここでyear関数が使えるなら

select year(create_datetime) as y,count(*) as c from tbl group by y;

さらに2016年だけ抜き出すなら

select year(create_datetime) as y,count(*) as c from tbl where 
year(create_datetime)='2016' group by y;
/*もしくは*/
select year(create_datetime) as y,count(*) as c from tbl  
group by y having y='2016';

たとえば2016年の上半期だけぬきたいなら

select year(create_datetime) as y,count(*) as c from tbl
where create_datetime between '2016-01-01 00:00:00' and '2016-09-31 23:59:59'
group by y;

interval とbetween を使って作成しました。 

--１年間
select count(*) FROM tm_user_e 
WHERE create_datetime::date between '2012-12-31' AND date '2012-12-31' +interval '1 year -1 day';
--２年間
select count(*) FROM tm_user_e 
WHERE create_datetime::date between '2012-12-31' AND date '2012-12-31' +interval '2 year -1 day';

--１か月間
select count(*) FROM tm_user_e 
WHERE create_datetime::date between '2012-12-31' AND date '2012-12-31' +interval '1 month -1 day';
--２か月
select count(*) FROM tm_user_e 
WHERE create_datetime::date between '2012-12-31' AND date '2012-12-31' +interval '2 month -1 day';

--２日間
select count(*) FROM tm_user_e 
WHERE create_datetime::date between '2012-12-31' AND date '2012-12-31' +interval '2 day -1 day';
select count(*) FROM tm_user_e 
WHERE create_datetime::date between '2012-12-31' AND date '2012-12-31' +interval '1 day';

--３日間
select count(*) FROM tm_user_e 
WHERE create_datetime::date between '2012-12-31' AND date '2012-12-31' +interval '3 day'-1 day';

--１年９か月間
select count(*) FROM tm_user_e 
WHERE create_datetime::date between '2012-12-31' AND date '2012-12-31' +interval '1 year 9 month -1 day';