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Update memory.md
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@ -22,8 +22,7 @@ Normally, using this table engine is not justified. However, it can be used for
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The Memory engine is used by the system for temporary tables with external query data (see the section “External data for processing a query”), and for implementing `GLOBAL IN` (see the section “IN operators”).
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Upper and lower bounds can be specified to limit Memory engine table size, effectively allowing it to act as a circular
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buffer (see [Engine Parameters](#engine-parameters)).
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Upper and lower bounds can be specified to limit Memory engine table size, effectively allowing it to act as a circular buffer (see [Engine Parameters](#engine-parameters)).
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## Engine Parameters {#engine-parameters}
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@ -40,9 +39,6 @@ buffer (see [Engine Parameters](#engine-parameters)).
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## Usage {#usage}
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``` sql
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CREATE TABLE memory (i UInt32) ENGINE = Memory;
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```
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**Initialize settings**
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``` sql
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@ -56,16 +52,42 @@ CREATE TABLE memory (i UInt32) ENGINE = Memory SETTINGS min_rows_to_keep = 100,
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CREATE TABLE memory (i UInt32) ENGINE = Memory SETTINGS min_bytes_to_keep = 4096, max_bytes_to_keep = 16384;
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/* 1. testing oldest block doesn't get deleted due to min-threshold - 3000 rows */
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INSERT INTO memory SELECT * FROM numbers(0, 1600);
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INSERT INTO memory SELECT * FROM numbers(0, 1600); -- 8'192 bytes
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/* 2. adding block that doesn't get deleted */
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INSERT INTO memory SELECT * FROM numbers(1000, 100);
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INSERT INTO memory SELECT * FROM numbers(1000, 100); -- 1'024 bytes
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/* 3. testing oldest block gets deleted - 9216 bytes - 1100 */
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INSERT INTO memory SELECT * FROM numbers(9000, 1000);
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INSERT INTO memory SELECT * FROM numbers(9000, 1000); -- 8'192 bytes
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/* 4. checking a very large block overrides all */
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INSERT INTO memory SELECT * FROM numbers(9000, 10000);
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INSERT INTO memory SELECT * FROM numbers(9000, 10000); -- 65'536 bytes
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SELECT total_bytes, total_rows FROM system.tables WHERE name = 'memory' and database = currentDatabase();
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```
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``` text
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┌─total_bytes─┬─total_rows─┐
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│ 65536 │ 10000 │
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└─────────────┴────────────┘
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```
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also, for rows:
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``` sql
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CREATE TABLE memory (i UInt32) ENGINE = Memory SETTINGS min_rows_to_keep = 4000, max_rows_to_keep = 10000;
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/* 1. testing oldest block doesn't get deleted due to min-threshold - 3000 rows */
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INSERT INTO memory SELECT * FROM numbers(0, 1600); -- 1'600 rows
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/* 2. adding block that doesn't get deleted */
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INSERT INTO memory SELECT * FROM numbers(1000, 100); -- 100 rows
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/* 3. testing oldest block gets deleted - 9216 bytes - 1100 */
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INSERT INTO memory SELECT * FROM numbers(9000, 1000); -- 1'000 rows
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/* 4. checking a very large block overrides all */
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INSERT INTO memory SELECT * FROM numbers(9000, 10000); -- 10'000 rows
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SELECT total_bytes, total_rows FROM system.tables WHERE name = 'memory' and database = currentDatabase();
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```
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