Added demo for new article

Author: Ivan Vakhrushev

.gitignore

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+/.idea/

deduplication_of_btree_indexes_demo/initdb.sql

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+create table if not exists test
+(
+  id bigserial primary key,
+  fld varchar(255),
+  mark varchar(255),
+  nil varchar(255)
+);
+
+insert into test
+select data.id, case when data.id % 2 = 0 then now()::text else null end, case when data.id % 2 = 0 then 'test_string'::text else null end, null
+from generate_series(1, 100000) as data(id);
+
+create index if not exists i_test_fld_with_nulls on test (fld);
+create index if not exists i_test_fld_without_nulls on test (fld) where fld is not null;
+create index if not exists i_test_mark_with_nulls on test (mark);
+create index if not exists i_test_mark_without_nulls on test (mark) where mark is not null;
+create index if not exists i_test_nil_with_nulls on test (nil);
+create index if not exists i_test_nil_without_nulls on test (nil) where nil is not null;

deduplication_of_btree_indexes_demo/readme.md

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+# De-duplication of B-tree indexes in PostgreSQL 13
+
+## Run PostgreSQL 13.2 in Docker
+```
+docker run --name postgres-13 -e POSTGRES_USER=testuser -e POSTGRES_PASSWORD=testpwd -e POSTGRES_DB=testdb -d -p 5432:5432 -v /absolute/path/to/initdb.sql:/docker-entrypoint-initdb.d/initdb.sql postgres:13.2
+```
+
+## Run PostgreSQL 12.6 in Docker
+```
+docker run --name postgres-12 -e POSTGRES_USER=testuser -e POSTGRES_PASSWORD=testpwd -e POSTGRES_DB=testdb -d -p 6432:5432 -v /absolute/path/to/initdb.sql:/docker-entrypoint-initdb.d/initdb.sql postgres:12.6
+```
+
+## Run psql from Docker CLI
+```
+psql -U testuser -d testdb
+```
+
+## Get indexes size
+```sql
+select
+  x.indrelid::regclass as table_name,
+  x.indexrelid::regclass as index_name,
+  pg_size_pretty(pg_relation_size(x.indexrelid)) as index_size
+from pg_index x
+join pg_stat_all_indexes psai on x.indexrelid = psai.indexrelid and psai.schemaname = 'public'
+order by 1,2;
+```
+
+### PostgreSQL 12 results
+|table_name |        index_name         | index_size | percentage
+|-----------|---------------------------|------------|-----------
+|test       | test_pkey                 | 2208 kB    | -
+|test       | i_test_fld_with_nulls     | 3552 kB    | -
+|test       | i_test_fld_without_nulls  | 2456 kB    | 30% less
+|test       | i_test_mark_with_nulls    | 2664 kB    | -
+|test       | i_test_mark_without_nulls | 1568 kB    | 41% less
+|test       | i_test_nil_with_nulls     | 2224 kB    | -
+|test       | i_test_nil_without_nulls  | 8192 bytes | -
+
+### PostgreSQL 13 results
+|table_name |        index_name         | index_size | percentage
+|-----------|---------------------------|------------|-----------
+|test       | test_pkey                 | 2208 kB    | -
+|test       | i_test_fld_with_nulls     | 704 kB     | -
+|test       | i_test_fld_without_nulls  | 368 kB     | 47% less
+|test       | i_test_mark_with_nulls    | 696 kB     | -
+|test       | i_test_mark_without_nulls | 360 kB     | 48% less
+|test       | i_test_nil_with_nulls     | 696 kB     | -
+|test       | i_test_nil_without_nulls  | 8192 bytes | -